Doubt is not a pleasant condition, but certainty is absurd - Voltaire

I think the categorical imperative is a type of certainty, that some actions should never be done and the case in the philosophy of law, but perfect certainty is complex least of all in the sciences. With regards to indubitability, I agree with the legal sentiment 'beyond reasonable doubt' vis-a-vis certainty, thus Klein' view that it needs to be both subjectively and objectively immune to doubt.

Nevertheless, it could all be psychological in nature and the latter is quite simply unsatisfactory in my opinion. I am certain that right now I have a migraine - even though I have never had one before and I have no idea why I have one today - yet, right now I am certain that my head hurts. I cannot guarantee a sufficient explanation to prove my migraine and thus will always rest on probabilities. There is no justification we can make other than through a historic approach by selecting the most reliable belief as truth with certainty.

Right @TimeLine, to you the bare existence of those experiences is certain, albeit perhaps not quite what they are (quiddity).
I guess that's not far off the Cartesian cogito ergo sum.

Descartes is in dangerous waters at this point, for if indeed the only claim that is indubitable here is the agent-independent claim that there is cognitive activity present, then he can be fairly associated with Averroist panpsychism, and its considerable taint. At a minimum, the argument requires a significant leap of reasoning, and for Gassendi, this is further evidence that Descartes places altogether too much faith in his criterion and the work he thinks it can do.

_{Source: Pierre Gassendi, Stanford Encyclopedia of Philosophy} — Saul Fisher

In short, if we're to stick with error-free deductive certainty, then Descartes took one step too many by injecting "I" (self).
The deduction derives mere existence of some experiences and awareness, which gets us roughly to some radical solipsism (as if not all solipsism is radical :)), and no further.

Like lots of these questions, to me certainty depends on who wants to know and why. In that sense, propositional certainty just has degrees. Are you certain you're going to the theatre on Tuesday? Are you certain the rope from which I'm dangling is securely fastened to an immovable rock? 'Certain' seems entirely useable in both these contexts but to have rather different meanings.

I am certain that right now I have a migraine - even though I have never had one before and I have no idea why I have one today - yet, right now I am certain that my head hurts. — TimeLine

How can you be certain that what you have is a migraine then? You might be quite sure that your head hurts, but a migraine is a certain type of headache. From what you've described, it doesn't sound like you're at all certain about having a migraine

If Smith truthfully asserts "I am certain that P" or "P is certain," what is anyone else to make of such assertions other than (1) Smith's mental or psychological state is such that he has no doubt that P, and/or (2) Smith has determined that P is warranted by whatever explicit or tacit epistemic standards he deemed sufficient?

It seems to me that what the rest of us normally care about is whether or not we also should accept P (given that we care about the truth or falsity of P at all.) And whether we also accept P is determined by whether or not we ourselves judge there to be sufficient warrant for P. The weight we assign to Smith's endorsement of P would vary considerably depending on the context--such as our trust in Smith's judgment about such a thing as P, including what we may know about the standards he applied.

In the end, I think someone's reported "certainty" amounts to little more than playing a rhetorical role (conveying their strong endorsement of the proposition at issue), and perhaps implying that a certain tacitly understood epistemic standard was satisfied.

...Descartes took one step too many by injecting "I" (self). — jorndoe

Peculiar how the 'I' was supposed to establish certainty :-}

How can you be certain that what you have is a migraine then? You might be quite sure that your head hurts, but a migraine is a certain type of headache. From what you've described, it doesn't sound like you're at all certain about having a migraine — Metaphysician Undercover

I'm not certain, hence reliabilism. I have never had a migraine before but yesterday I woke up with a sharp pain in my head, my eyes were sore and I felt unwell. Similar symptoms have occurred in others who have had migraines, so I can only assume that it must be the case that I too had a migraine. My only certainty was that I felt pain in my head, but was it even pain, and in addition I did not have any of the preceding underlying causes i.e. sleeplessness, stress, so the 'migraine' was quite random. I understand the possibility of an infinite regress of reliable causation because while we may have similar perceptions of experience it does not necessary make it identical.

Probably not the best example, but I cannot agree that my belief that I have a migraine is epistemically justifiable, though the product of my experience of pain in the head can perhaps be articulated. In the end, as jorndoe pointed out: to you the bare existence of those experiences is certain, albeit perhaps not quite what they are (quiddity). At base level, it is a mind/body problem or conception and cognition, but think of pseudocyesis and it is easy to see why psychological certainty is never reliable.

I am interested in how to define universal notions (I mentioned the categorical imperative) and I think Spinoza' scientia intuitive in his ethics is interesting in that the types of cognition must equally balance between belief, reason and intuition [subjective and objectively], though I fear the result of his certainty may rest on the idea that 'cognition depends on the knowledge of its cause' and that - similar to the cartesian angle - fundamentally attributed to God (or nature).

I am interested in how to define universal notions (I mentioned the categorical imperative) and I think Spinoza' scientia intuitive in his ethics is interesting in that the types of cognition must equally balance between belief, reason and intuition [subjective and objectively], though I fear the result of his certainty may rest on the idea that 'cognition depends on the knowledge of its cause' and that - similar to the cartesian angle - fundamentally attributed to God (or nature). — TimeLine

I don't agree with the categorical imperative, I think it is a fiction. I think that every good is particular, determined relative to the specific circumstances, and therefore assuming such a universal good is a category mistake.

This is the reason why absolute certain is beyond reality. Every application of mathematics, or any type of logic, is applied to a particular situation. The application of logic is seen as the good for understanding the situation. But all the particulars of any situation will never be grasped. So in applying the universal to the particular there will always be something not completely covered, and this will create a degree of uncertainty.

To say that there is certainty within the logical principles, without being applied, such as certainty that 2+2=4, is meaningless because such certainty would be symbols without content. If we say that there is certainty as to what 2 means, and what + means, then we have gone to a completely subjective certainty, with a low degree of certainty.

I don't disagree with you, but there needs to be some line drawn otherwise we will find ourselves in an even more absurd position, namely solipsism.

I cannot tell with certainty whether someone truly loves me but I calculate according to a number of historic and logical inferences to trust that his description of love must be an accurate description of his feelings. I have nothing to offer, no family, no popularity or many friends, no money, no capacity to have children, heck, if I were to get married there would be a tiny handful of people on my side of the church. I have experienced a history of negative misuse of my social vulnerability from people who care about their social position, so a declaration of love in addition to my testing the authenticity of his expressions for me must logically mean that there is no other reason for his love other than 'me' since I have nothing else to offer. It is a completely subjective certainty based on the authenticity of the symbols he uses to describe what he feels and since I feel a similar way vis-a-vis his articulation, I trust that he must love me. I will never know for certain, but again, you need to draw the line somewhere before things start to get absurd.

In the end, though, it could simply be psychological based on my own existential vulnerability and therefore remains unreliable. But since I feel, at the very least I am capable of expressing this feeling and that should suffice; things change when you take out the 'I' and start to just live, even if it is all a dream-like vanity state of mind.

I don't think we can even have context-based certainty like a short mathematical proof, and even the Cartesian certainty of existence.

Because our knowledge of the truth of those propositions is based on our remembering having mentally worked through the proof, and on correctly remembering the meaning of the words used. How can we be certain that our memory of working through the proof is valid, and how can we be sure we didn't miss a subtle flaw?

So I find it simpler to just approach knowledge on the basis of 'degrees of confidence'. There is nothing at all of which I can be completely confident. But I can be confident of some things more than others, and for some - like maths proofs and the occurrence of thought - I am very confident indeed.

I don't disagree with you, but there needs to be some line drawn otherwise we will find ourselves in an even more absurd position, namely solipsism. — TimeLine

Why would you think that a belief in the impossibility of absolute certainty would lead to solipsism? I would think that since certainty is the property of the subject, the one who is certain, then solipsism would be more closely related to certainty than to skepticism. Being certain would create a division between that which is within one's own mind, which the individual is certain of, and all that is outside one's mind, which one cannot be certain of. If we allow that there is not even certainty within our own minds, then we produce a unity, or consistency, between what is inside the mind and what is outside, both being uncertain.