Essential attributes and accidental attibutes are both properties, and properties are universals. — Mitchell
What is more important here is that the accidental and the essential (or the necessary) are a dialectical dichotomy - defined as a mutually exclusive and jointly exhaustive pair of complementary terms. And so through the unity of opposites, we can track their emergence from pure possibility (vagueness) as each other's mirroring generality.
In the view I am taking, all universals are not singular but dual. True generality is defined by matched pairs that speak to opposing extremes of realised being.
So if I say form, you say matter. If I say discrete, you say continuous. If I say chance, you say necessity. If I say flux, you say stasis. And so on through all the fundamental ontologically-basic categories.
Once the duality, or complementary logic, of "metaphysical strength" universals is accepted, then it becomes obvious that the two need the third thing of a ground of common being from which they can arise as the opposed limits of "the possible". They point back to their own grounding in vagueness - pure potentiality - by the very fact that they represent the counterfactual extremes on that possibility.
For the discrete to be distinct from the continuous - for that distinction to be actualised in a general way by a world - then each must successfully exclude all possibility of its "other". Thus also, the possibility of the "other" must exist to actually get excluded in this fashion. The potential is defined (in dichotomous contrast to the actual!) as then the state where nothing is yet excluded. So before the dichotomy can be the case, there must be the third thing of a vagueness that is neither discrete nor continuous in any degree ... a state that is just the potential for such a division to arise.
Of course, not all "universals" are metaphysical strength dichotomies. That is where a lot of confusion starts. Whiteness is a reasonably strong universal, in being the complete opposite of blackness. But a horse is really a fairly particular "universal".
The concept of "a horse" sort of excludes donkeys and mules and zebras. But the boundaries are vague. And more importantly "a horse" does not stand completely opposed to any other generality. A horse is a living organism as opposed to an inanimate object. A horse partakes of more fundamental metaphysical dichotomies. It is a continuous whole in terms of itself, a discrete part in terms of its world. But even here, the boundaries of the concept of "a horse" remains vague. Is the sweat that is about to drop off the horse still part of its structural continuity or now part of what counts towards defining its structural discreteness?
The problem with the conventional take on universals is that people try to reason about them using the logic of particulars - the predicate logic secured by the three laws of thought. Peirce unpicked that logic by showing that generality is rightfully defined by the LEM failing to apply, and vagueness by the PNC failing to apply.
This opens up the system of reasoning so that we can see that what defines the metaphysically fundamental categories is the absolute division they achieve via dialectical opposition. The discrete and the continuous mutually define the extremes of a certain kind of universal possibility. And in defining the extremes, they together point back to the undivided potential that must have been there to birth them.
Aristotle of course tackled this in his Organon in contrasting contradiction and contrariety.
Two statements are contradictory if one affirms or denies universally what the other affirms or denies particularly. But two statements are contrary if one affirms or denies particularly what the other affirms or denies particularly of the same thing.
So a contradiction excludes a middle, but a contrariety admits to a middle. The same substance - your soup - could be hot, or it could be cold, or it could be anywhere in between. If all soup is hot, then it is a contradiction if your soup is cold. But if my soup is hot, then it is only a contrary fact that yours is cold.
The trick then is to see that when we speak of universals as the product of dichotomies or dialectical opposition, we are now contrasting two particulars. They are only contrarieties (as if they were contradictions, then one couldn't even be considered a possibility, and if one wasn't possible, then its "other" can't even be crisply defined).
If the poles of a dichotomy are only contraries, then only the vague, or pure potentiality, counts as some actual monistic universal - but now an apophatic one, defined by its actual non-existence.
What actually exists is not the "oneness" of this potential but the "many-ness" of the divisions that proved to be possible. So universality unfolds hierarchically in the manner first articulated by Anaximander (well, metaphorically by the Hesiod also). That is, metaphysics seeks to identify the most general dichotomies (or symmetry breakings) that then led to the increasingly more specific ones.
Anaximander actually managed a strikingly thermodynamic view. The Apeiron first separates into the hot and the cold, then follows the division into the dry and the wet. Earlier I highlighted the fundamental division that modern physics appears to have arrived at - the two basic "directions of action" of a (gravitational) integration and a (quantum) differentiation.
But whatever the story, the logic is the same. A first most general symmetry breaking paves the way for a casade of further symmetry breakings. Universality has hierarchical organisation - an unfolding direction in time. The most general change sets the scene for more particular change. And every metaphysically significant change takes the logical form of a dichotomisation.
Hence why hylomorphic substance was taken by Aristotle to be the foundational state of being. The combo of en-mattered form and in-formed matter. Everything definitely begins only when material cause and formal cause are brought together in an actualised state of contrariety - when they are realised with the middle ground that connects them as the matched extremes of a state of possibility.
But then when did these two "actions in directions" first arise? The talk of prime matter and prime mover tries to bypass this question by just claiming their actuality, their divided particularity, as something static or eternal. So again, to get beyond the usual impasse, we have to have a triadic metaphysics where the grounding potential is defined apophatically in terms of what we consider to be the most fundamental pair of universals. If this is form and matter, then that is what a vague beginning swallows up.
Substance can't be the substrate of substance. But a potential could be divided in any fashion by a dichotomisation. And logically - reversing the hierarchical story to be seen in metaphysical development - the first dichotomy is then going to be whatever is the most general possible one. If generality has to set the scene for specificity, then that is why Anaximander (or the Hesiod) intuitively sought the most general possible dichotomy as the first act of world-creating symmetry breaking. And - back to modern physics - why a theory of quantum gravity seems fundamental to explaining the "how" of a Big Bang creation event.