• Eros1982
    32
    Dear philosophers,

    A government employee in his late thirties made a big mistake of enrolling in an MA program in Philosophy (without any previous experience at all), assuming that he could succeed in this program through devoting most of his hours to work and sleep, not to philosophy. This program wasted two years from his life and now he is in the difficult position to have failed the logic exam (although he defended his MA thesis) and not being able to leave work in order to take the logic class that could be an alternative to the exam. Is anyone here willing to help with some basic (and maybe easy for you) questions which are not so clear in the literature professors assigned to him (me) for this exam?

    I definitely will google first before I bother the members of this forum, but sometimes my questions are so specific and Google seems too big for them.

    My first two questions about natural deduction (proofs):

    1) Can I introduce a sequent (as Modus Tollens, Hypothetical Syllogism, etc.) to previous lines which are still quantified, or should I always get rid of the quantified variables (with new lines with proxies/substitutes) before I introduce sequents?

    2) Is it possible to use an assumption in the discharge of another assumption? (I guess that this would be an error, but I am little confused from the exercises I have been given).

    I appreciate all your support, and I look forward to contribute in non-logic discussions of this forum :)

  • Nagase
    132
    It'd be nice if you provided the specific rules that you are working with. This may sound surprising, but there are a lot of natural deduction systems out there, each one with its own idiosyncrasies, so it'll be hard to help without seeing the specific system with which you are working. Some examples of what you are attempting to do would be nice, too. That said, I'd say that at least with regards to question 2, the answer is probably yes.
  • Eros1982
    32
    Ok, I appreciate your help. This is the example I am trying to resolve (from G. Forbes' book, p. 268, no answer is given though the author says there are 4 solutions/symbolizations).

    With "Ax" are meant universal quantifiers, and with "Ex" existential quantifiers.

    (Ax)[(Ey)Tyx--then-->(Az)~Txz], (Ax)[(Ey)Tyx--then-->Txx] |--NK (Ax)(Ay)~Txy

    Thank you so much!!!
  • Eros1982
    32
    Ok here is the solution I "found", without any sequent introductions... There seem to be many assumptions and I hope I have discharged them all.

    1) (Ax)[(Ey)Tyx-->(Az)~Txz] premise
    2) (Ax)[(Ey)Tyx-->Txx premise
    3)(Ey)Tya-->Taa 2AE (viz. universal elimination of 2)
    4) Tba-->Taa assumption
    5)Tba assumption
    6) Taa 4,5 -->E
    7)(Ey)Tya-->(Az)~Taz 1AE
    8) Tba-->(Az)~Taz assumption
    9)Tba assumption (I assume it a second time since I am not sure if I should use line 5, already discharged, again)
    10) (Az)~Taz 8,9 -->E
    11) ~Taa 10AE
    12) ^^^contradiction^^^ of 6 and 11
    13) ~Tba 5,12~I
    14) (Ay)~Tby 13AI
    15) (Ax)(Ay)~Tby 14AI
    16) (Ax)(Ay)~Tby 3,4,15 EE (viz. existential elimination)
    17) (Ax)(Ay)~Tby 7,8,16 EE

    By the way, these exercises do not help me pass the exam. I have to resolve them in order to make sense of everything and be able to interpret all the problems I will be given in the exam. Hence, I'd appreciate any corrections in my argument. Thank you again!
  • Eros1982
    32
    Nagase forgot this discussion :worry:

    Anyone else willing to help?

    Thanks!
  • jkg20
    225
    You discharge assumption (4) at step (6) by relying on assumption (5). You discharge assumption (8) at your step (10) but again you should be relying on (5) to do the discharging, you should not introduce (9). Since you now get your contradiction, and you only have assumption (5) left undischarged, you can discharge it by negating it. Does that help?
  • jkg20
    225
    So, up to (8) everything is fine (except you forgot to close the brace at (2) - I know its nitpicking, but that's what formal logic requires). After (8) you should have:
    (9) (Az)~Taz
    (10) ~Taa
    After that it depends what system of rules you are using, but when I learnt logic you would then do
    (11) Taa & -Taa
    (12) -Tba

    The system I learnt also required that one list explicity at the end of each line which premises and assumptions were still "in play" in arriving at the line itself, which you are not doing above, but as Nagase says, it depends on the rules you've been given.
  • Eros1982
    32
    jkg20

    Insofar as everything depends on the details, I think you have been helpful already. My weakest point are assumptions (and I am confused with many braces and quantifiers as well). In the same book I am asked to show that |--NK (Ax)(Ay)((Fx & ~Fy)-->(x≠y) and again I am a little confused on whether (x≠y) can be equal to ~(x=y).

    I guess in order to get such a conclusion I should start with: premise 1) Fa & ~Fb, premise 2) ~(a≠b). With the second premise I don't know what kind of proof or rules I may use (the chapter is referred to identity elimination rule =E), but I am not sure if premise 2 can be (a=b) --hoping to bring its opposite through introducing a negation (since I really do not know other way to turn = to ≠ in a proof).

    Any ideas/suggestions on this last question? (I will try to solve it by myself also and hopefully will keep you updated).

    Thank you for your support!
  • Eros1982
    32


    As I said, all help is appreciated for a beginner like me... and details are really important. Below I bring the second proof that gave me some hard time, since I am not sure what is the exact route of turning "=" into "≠". In the book I am given only the conclusion, no premises, and I try to solve it with some legitimate assumptions. I would greatly appreciate it if you or anyone else takes a little time with my solution to the exercise : Show |--NK (Ax)(Ay)((Fx & ~Fy)-->x≠y)

    My proof:
    1) Fa & ~Fb................................assumption
    2) a=b..........................................assumption
    3) Fa.................................. ........1&E
    4) ~Fb................................ ........1&E
    5) Fb............................................2,3 =E
    6) ^^^contradiction^^^^.................. 4,5~E
    7) ~(a=b)......................................2,6 ~I
    8) (Fa & ~Fb)---> ~(a=b)....... ......1,7-->I
    9) (Ay) ((Fa & ~Fy)--->a≠y).... ......8 AI -(as I said I am not sure how "=" turns into "≠" in a proof)
    10) (Ax)(Ay)((Fx & ~Fy)---> x≠y)... 9 AI
  • dclementsAccepted Answer
    269
    "Ok here is the solution I "found", without any sequent introductions... There seem to be many assumptions and I hope I have discharged them all.

    1) (Ax)[(Ey)Tyx-->(Az)~Txz] premise
    2) (Ax)[(Ey)Tyx-->Txx premise
    3)(Ey)Tya-->Taa 2AE (viz. universal elimination of 2)
    4) Tba-->Taa assumption
    5)Tba assumption
    6) Taa 4,5 -->E
    7)(Ey)Tya-->(Az)~Taz 1AE
    8) Tba-->(Az)~Taz assumption
    9)Tba assumption (I assume it a second time since I am not sure if I should use line 5, already discharged, again)
    10) (Az)~Taz 8,9 -->E
    11) ~Taa 10AE
    12) ^^^contradiction^^^ of 6 and 11
    13) ~Tba 5,12~I
    14) (Ay)~Tby 13AI
    15) (Ax)(Ay)~Tby 14AI
    16) (Ax)(Ay)~Tby 3,4,15 EE (viz. existential elimination)
    17) (Ax)(Ay)~Tby 7,8,16 EE

    By the way, these exercises do not help me pass the exam. I have to resolve them in order to make sense of everything and be able to interpret all the problems I will be given in the exam. Hence, I'd appreciate any corrections in my argument. Thank you again!"
    - Eros1982

    Sometimes I'm so glad that I have never taken a logic in philosophy course or something like it since it would likely make me hate the entire subject and make me never what to do it again for the rest of my life. When I was studying electronics I LOVED doing things like digital circuits, but it didn't have half the madness that logic entails.

    While it is unlikely you will heed this advice, IMHO I believe it is best that you reading and writing about philosophy other than what you are doing to earn your degree on your own terms and doing what interests you. For if you don't there is a good chance that it will all will become just 'work' to you and it will become just as boring and tedious as whatever you did before you did for the government before you enrolled in the program you are now in.
  • jkg20
    225
    OK, Eros, first thing is that after taking a quick look at the way Forbes gives his answers, each line of your answers need to be structured in the folloing way:
    a/p1, a/p2, .... ( N ) Statement l1, l2... L/P1 L/P2...
    a/p1 on the left refers to the undischarged assumptions or premises that support the Statement. N) is just the line number of the argument. l1,l2 refer to the preceding line numbers of the argument you are giving that are being used to make the Statement, and L/P1... to the logical laws/statement types you are availing yourself of or making. If you don't follow that form in all your answers you will be penalized in your examination. Depending how strict your examiners are you may not get a single mark: "almost right" doesn't cut it in formal logic. Here is a simple example using propositional rather than predicate logic, but which will give you the basic idea:
    1 (1) A → B Premise
    2 (2) A→ C Premise
    3 (3) A Assumption
    1,3 (4) B 1,3 →E
    2,3 (5) C 2,3 → E
    1,2,3 (6) B & C 4,5 &I
    1,2 (7) A → (B &C) 3,6 →I

    So take a look at line (6) for instance. ON the right we see that we are using steps 4 and 5 of the argument plus the law of conjunction introduction. However, on the left we must list the assumptions or premises that allow us to make the statement, and here we are relying on the two premises we introduced and the assumption we made, lines 1, 2 and 3. At line (7) we use the implication introduction rule plus lines (3) and (6), but since the implication introduction rule tells us that in introducing an assumption as the antecedent of an implication, we discharge that assumption, on the left we now only need to list (1) and (2) as the steps of the argument we are still relying on (i.e. just our two initial premises).

    Exactly the same principles apply to arguments in predicate logic, but you have extra/differently expressed laws of inference (universal and existential introduction/elimination for instance). You need to get into the habit of using precisely the same format in your answers.

    I was a little confused as to why you had a separate line to state the contradictions you draw, but I see from Forbes's examples that this is how he has specified the Law of Non Contradiction - the system I used didn't require a separate line for stating the contradiction, you could simply refer back to the individual lines where the two contradictory statements were initially made. Anyway, that changes nothing, it's a matter of taste, but since you will be marked according to Forbes's system, you should continue to do as you are doing and express the contradiction explicitly on its own line (although I would use the actual symbol Forbes introduces for this, which looks like an elaborate "^" . If you want to do that in any subsequent posts, go ahead, I'll understand.)

    As for getting from -(a=y) to a≠y, you need to look at exactly how Forbes deals formally with the law of identity, my guess is that he defines somewhere a rule that allows you to use the sign for non-identity (≠) as logically equivalent to the negation of an identity -(x=y). I don't have a copy of the book, and the excerpts I can get online are limited, so you are on your own there I'm afraid.

    So, first off, skim the Forbes to see what he has to say about the law of identity, and then redo both your solutions in the format that Forbes requires. Then we can see if there are any other issues to address.
  • Eros1982
    32


    Ok, thank you so much for your time. You have been helpful, although I will reread your reply since the numbers on the left really confuse me. I will also ask my professor if I can avoid those numbers at the time of the exam. Unfortunately, I can't attend the logic class (since I can't leave work three times a week), and Forbes does not explain every detail in his book (I am a good reader, believe me).

    By the way for proofs where I have to show that x=y, I suppose that x and y should pick the same referent. In other words, in a premise like (Ax)(Fx-->(Ey)(Gy & x=y)), I guess it is legitimate and helpful to substitute Fx with Fa and Gy with Ga (where both F and G predicates pick the same referent "a" and it becomes easy for me to show that x=y). Right?

    Thank you again!
  • jkg20
    225
    You're welcome - actually enjoyed dredging up memories of undergraduate mathematical logic (I was pretty good at it back in the day :wink: ). If you are confused about the numbers on the left, ask me a question about them - you might find that in framing the question you arrive at the answer yourself, but if not, I'll be happy to respond as best I can.

    As to your second paragraph above, I'm not quite sure what you are asking me, sorry. If you have this premise :
    (Ax)(Fx-->(Ey)(Gy & x=y))
    You can derive
    (Fa-->(Ey)(Gy & a=y))
    by univeral elimination. Universal elimination allows you to substitute a constant term or name ('a') for a variable (in this case 'x') wherever that variable is bound by the universal quantifier. However, you cannot get from
    (Fa-->(Ey)(Gy & a=y))
    to
    (Fa-->Ga & a=a))
    by making the same substitution - no rule of inference I know of would warrant that step.
  • Eros1982
    32


    Professor said that I can avoid the dependency numbers (on the left) in my exam :) This is good news, because when you have many assumptions in a proof you have to take your time looking which ones are not discharged yet in order to enlist them on the left (with the other numbers where the current line depends). The are almost 20 questions to be answered in two hours, and the more time I save the better :) I may come up with a few more issues in the near future.

    Everyone enjoy your weekend!
  • Eros1982
    32


    Ok I resolved the last exercise by myself, it turned to be easy.

    Now there is one more conclusion. This is what I should prove (through using =E or =I rules):
    |--nk (Ax)(x=b --> x=c)-->b=c

    One way of proving this is...
    1) (Ax)(x=b --> x=c)............. assumption
    2) (a=b)--> (a=c)................ 1AE
    3) a=b................................ assumption
    4) a=c.................................2,3 -->E
    5) b=a....... ........................3 COM (law of commutation)
    6) b=c................................4,3 =E
    7) (Ax) (x=b-->x=c)-->b=c

    Does it look good?

    I hope I am not boring you --you said you enjoyed memories from your undergraduate years (before you turned into a scientist I guess :), and I am trying all the available means in order not to fail again in the logic exam...
  • jkg20
    225
    The problem as I see it with your answer is that you have the undischared assumption at line (3) still lingering around by the time you get to line 7. I think one way of doing might be as follows
    1 (1) a=b Assumption
    2 (2) a=c Assumption
    1,2 (3) b=c 1,2 IE
    2 (4) (a=b -> a=c) 1,2 ->I
    (5) (a=b -> a=c) -> b=c 4,3 ->I
    (6) Ax(x=b -> x=c)-> b=c 5, AI

    I discharge all my assumptions by line (5) using conditional introduction, and since (5) depends on no undischared assumptions or any premises, I can use universal introduction to replace all occurrences of a.
  • jkg20
    225
    As an aside, one good general technique when what you are aiming to prove is a conditional statement, you should be looking to make the antecedent of the conditional an assumption, and the negation of the consequent your second assumption, and then look to draw a contradiction. But in this case that doesn't seem necessary.
  • Eros1982
    32


    It seems I am not well acquainted with discharging assumptions, and that's the real issue here. I thought that I had discharged assumption of line 3 on line 4, where I apply existential elimination rule on lines 2 and 3... but you say that 3 is not discharged. I will google more about dischrging assumptions, but it would be helpful if you could tell me why line 4 is not considered a discharge of the assumption in line 3. Can you do that?

    Thank you again for your time!!!
  • jkg20
    225
    Hi. Discharging assumptions basically means that you are no longer relying on the "fact" that they are true. Let's try putting your proof into "natural language". You have
    2) If John is Jane, then John is Janice. (by Universal elimination)
    3) John is Jane. (Assumption)
    Given assumption (3) is true, then you infer:
    4) John is Janice
    from the conditional elimination rule applied to the conditional (2).
    This is fine. However, the conditional elimination rule does not allow you to discharge assumptions. In using that rule you are in effect relying on the truth of the antecedent (in this case that John is Jane). So, the statement (4) that John is Janice is relying on your assumption that John is Jane.

    There are three principle ways of discharging assumptions: conditional introduction, negation and existential elimination. I won't deal with the latter, as it is one of the more complicated inference rules, and to get an idea of what's going on with assumption discharge, we don't need to look into its deatils. So, for negation, if you introduce an assumption in your argument:
    John is Jane
    and then later on in the argument , one way or another - it doesn't matter how as long as you follow the rules - you arrive at the statement
    It is not the case that John is Jane (or more colloquially, John is not Jane)
    you have gotten rid of your initial assumption simply by "denying it".

    A more subtle way of discharging an assumption is by introducing it as the antecedent of a conditional. The very basic way of doing this would be the following:
    1) John is Jane Assumption
    2) If John is Jane, then John is Jane ->I
    Of course, you are not saying an awful lot in 2) - it's basically just an empty tautology - but never the less, you no longer are relying on the fact that John is Jane in order for (2) to be true.

    In my version of the argument above, I use the conditional introduction rule twice in order to discharge assumptions. Picking on this threesome of John, Jane and Janice, I have
    John is Jane Assumption
    John is Janice Assumption
    If John is Jane then John is Janice ->I
    In making that move I discharge my assumption that John is Jane and leave everything about my argument hanging on whether or not John is Janice.

    I hope that helps.
  • Eros1982
    32


    It helps a lot, although I have to familiarize with proving conditionals --and reread your previous replies. From what you say, I have a to find a way to support/prove the antecedent before I arrive at the consequent (something like Modus Ponens). The difficult part, which I need to familiarize with, is to support my antecedents through relying on the given premises or assumptions.

    Thank you!
  • jkg20
    225
    Conditional elimination rule is basically just another name for modus ponens, so yes, if you are going to use it, you will need to either assume the truth of your antecedent, or to have derived that antecedent from other premises/assumptions you have already made in your argument. The thing to remember is that it is not an assumption discharging rule of inference: any assumptions you still have in play up to the point you use modus ponens/conditional elimination, remain in play after you have used that rule. Although the "numbers on the left" are apparently not obligatory for your exam, they can be very useful for keeping track of what you are still assuming in your argument (in fact, that is really what they are there for) so whilst you are revising, it might be worthwhile trying to use them, even if in the exam you can take a shortcut and forget about them.
  • Eros1982
    32


    Regarding your solution... Do you mean identity elimination with IE on line 3? If yes, does IE allow us move b from the right of the first line to the left of the third line? I mean your proof looks good, but I need to know the rule that allows one change the place of b on line 3. Thanks!

    1 (1) a=b Assumption
    2 (2) a=c Assumption
    1,2 (3) b=c 1,2 IE
    2 (4) (a=b -> a=c) 1,2 ->I
    (5) (a=b -> a=c) -> b=c 4,3 ->I
    (6) Ax(x=b -> x=c)-> b=c 5, AI
    — jkg20
  • jkg20
    225
    Yes I do mean Identity Elimination, and its precise use for you will depend on exactly how the IE rule is defined by Forbes. However, as I was taught, the identity elimination rule basically just tell you that from a given identity statement
    a=b
    say
    you can change any occurrence of 'a' in any other statement in which 'a' occurse with an occurence of 'b', and it would also allow any occurence of 'b' in any other statement to be replaced with an occurence of 'a'. So, in my argument I use the identity a=b to replace the 'a' in a=c with 'b'

    If the IE rule you have been given requires that the subsitution made be sensitive to the exact order of the terms in the identity statement, then you may have to go through some additional steps of swapping the terms around, but the rules as I remember being taught them did not require this.
  • Eros1982
    32


    In ten days I have to take the exam and I still do not feel ready with everything....

    Here is a conclusion that I am supposed to invalidate without any premises being given.

    |≠ (Ex)(Ay)((Rxy & ~Ryx) --> (Rxx <-> Ryy)

    I thought as a solution a domain with two members D= [a,b], where extension R=[<a,b>, <a,a>], hoping in this way to make the antecedent (Rxy&~Ryx) true and the consequent of the conditional false (since I am supposed to invalidate the sentence above). The author, however, does not give a solution to this problem. He just says that it was invented by Mendelson (?) and it can be solved only with four objects in the Domain (D=a,b,c,d or D=0,1,2,3?).

    I am now wondering why the solution I gave with two objects will not do?
  • jkg20
    225
    That's a tough one. I guess you have to show that the negation of the formula can be true under some interpretation, which seems to be what you are getting at. What you have to show is that in some interpretation the following is true:
    AxEy((Rxy & -Ryx) & -(Rxx <-> Ryy))
    An interpretation with two objects a,b and the extension for R <a,b> , <a,a> would not do that (you can make all the substitutions to see for yourself that the above formula never turns out to be true). You'd then have to play around with a,b,c and the consequent possibilities for Ext R to see if you could get any further. I suppose if Forbes/Mendelson is correct you need at least four objects in the domain and an extension for R that is suitably restricted. Note that for -(Rxx <-> Ryy) to be true under an interpretation you need the truth values of Rxx and Ryy to be opposites of each other.
    To be honest, I doubt you'll have anything as challenging as this to deal with in your exam.
  • jkg20
    225
    @Eros1982Maybe I could have been a little clearer about why the formula AxEy((Rxy & -Ryx) & -(Rxx <-> Ryy)) is not true in the domain you give.
    The interpretation contains a and b, with extension for R <a,b> and <a,a>.
    Within the context of that interpretation, elimination of universal quantification allows us to infer
    Ey((Rby & -Ryb) & -(Rbb <->Ryy))
    The extension for R you give does not allow for (Rby & -Ryb) to be true no matter whether you replace y by a or by b, but those are the only two possible replacements in your interpretation. This means that in the interpretation you give, the universal claim made by the formula just is not true. Obviously, adding <b,a> and <b,b> to the extension of R is not going to help. So, there is the need to have more than two objects in the domain in order to allow for the necessary flexibility in the extension of R, in order that we can get the universal claim to be true for all possible substitution instances of x. If you reiterate this kind of reasoning my guess is you will end up showing that AxEy((Rxy & -Ryx) & -(Rxx <-> Ryy)) is true for a domain with four objects (a,b,c,d) and some appropriately chosen extension for R. Hope that helps.
  • Leesa Johnson
    4
    This forum is a good platform for learners. Thank you for posting.
  • jkg20
    225
    Happy to (try to) help:smile:
  • Eros1982
    32


    Thank so much for you for your time. I will reread your replies in the weekend when I am off, with the hope that my mind will be sharper lol :)

    This strategy of turning |≠(Ex)(Ay)((Rxy & ~Ryx) --> (Rxx <-> Ryy) into AxEy((Rxy & -Ryx) & -(Rxx <-> Ryy)) is mentioned by the author as well, but I am a little confused why turning (Ex)(Ay) to (Ax)(Ey) would somehow help to invalidate the first.
  • jkg20
    225
    It's basically to do with the negation of quantifiers. If I say everything is red , Ax(Rx), then the negation of that claim is equivalent to saying that there is at least one thing which is not red, i.e. Ex-(Rx). If I say that at least one thing is red, Ex(Rx), the negation of that claim is equivalent to saying that everything is not red, i.e. Ax-(Rx). So, negating the whole of a quantified statement is essentially the same as switching the quantifier type and negating the proposition governed by the quantifier (this can be proved in first order predicate logic, but I won't bore you with that). The same principle works when quantifiers are compounded, so that the negation of AxAy(Rxy) becomes ExEy-(Rxy), the negation of ExEy(Rxy) becomes AxAy-(Rxy), the negation of ExAy(Rxy) becomes AyEx-(Rxy) and the negation of AxEy(Rxy) becomes ExAy-(Rxy). Suppose for instance that R is the relation 'is uglier than'. AxAy(Rxy) says that everyone is uglier than everyone else, and its negation is simply that there are at least two people such that one of them is not uglier than the other. ExAy(Rxy) states that there is at least one person that is uglier than everyone else - i.e. there is at least one contender for the title "ugliest person of the year". The negation states there is no such person, which is equivalent to saying that everyone is such that we can find someone who is uglier than they are - basically, no matter who you choose, they are better looking than at least someone else. Formalised, then, -(ExAy(Rxy)) is equivalent to AxEy-(Rxy). Now, it doesn't matter whether we replace Rxy by some more complicated compound sentence (although it becomes more difficult to find "real world" examples). So, if we replace Rxy by ((Rxy & -Ryx) -> (Rxx<->Ryy)), we will still be able to infer from -(ExAy((Rxy & -Ryx) -> (Rxx<->Ryy))) to AyEx-((Rxy & -Ryx) -> (Rxx<->Ryy)). You know already from basic first order calculus that -(A->B) is logically equivalent to A & -B. So, replacing A with (Rxy & -Ryx) and B with (Rxx<->Ryy), we end up with the conclusion that -(ExAy((Rxy & -Ryx) -> (Rxx<->Ryy))) is logically equivalent to AyEx((Rxy & -Ryx) & -(Rxx<->Ryy)). Now the claim |≠(Ex)(Ay)((Rxy & ~Ryx) --> (Rxx <-> Ryy) is essentially saying that there is at least one interpretation in which (Ex)(Ay)((Rxy & ~Ryx) --> (Rxx <-> Ryy)) comes out false, i.e. where -((Ex)(Ay)((Rxy & ~Ryx) --> (Rxx <-> Ryy))) is true for at least one interpretation, which, by the foregoing reasoning is equivalent to saying that there is an interpretation in which AyEx((Rxy & -Ryx) & -(Rxx<->Ryy)) is true. The challenge is to find the simplest interpretation which does the job (and apparently it is an interpretation with four objects).
    Hope that helps.
  • Eros1982
    32


    I will reread your last reply tomorrow (when I will switch provability with demonstrating invalidity :)

    There is one more question I'd like to ask, and I hope it is not difficult to you...

    When there are two existential quantifiers stacked at the beginning of a premise, I first replace the first variable with a substituting instance (instead of Ex I assume "a"), and then the second variable (instead of Ey I assume "b"). I am not sure, however, what I am supposed to do when the existential quantifier I wish to substitute has a negation. For example: -(Ex)(Ey)(x≠y & (Fx & Fy)).


    Should I substitute -(Ex) with negations only, e.g. (Ey)(-a ≠ y & (-Fa & Fy)) --and after think of a Sequent/Theorem Introduction (if there is one) that turns -a ≠ y to a=y--, or is there any other way to deal with quantifiers which contain negations?


    Thank you!
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