A ‘quiddity’ is simply that which makes something the kind of thing that it is – excluding anything that isn’t constitutive of its identity as that thing. Example: equinity, whatever its content, is a quiddity by virtue of which a horse is a horse and not, say, a cow. In its meaning, equinity or horseness excludes any property that isn’t essential to horses qua horses, like e.g., being white, swift, a particular size, etc.
Now what is ‘existence’ (wujud) and its relation to quiddity? Does it belong to quiddities themselves or does it belong the individual things out there in the world that we experience daily, like that horse of over there in the barn? Similar to contemporary (Fregean-Russelian) analytic philosophers, suppose someone holds that ‘to exist is to instantiate some quiddity’. Example: the statement ‘a horse exists’ would mean ‘the quiddity equinity has an instance’. On this view, existence would be something like a non-constitutive attribute of quiddities; it would not belong or characterize individual objects out there in external reality, at least no primarily. The reason is because ‘instantiation’ itself concerns quiddities, not individuals – e.g., the statement ‘Dhuljanah is instantiated’ or ‘has an instance’ makes no sense but ‘equinity has an instance’ (i.e., Dhuljanah) makes perfect sense. Primarily, therefore, ‘existence’ would be a property of quiddities, though not a property included in their very definition. This is arguably one way to understand the claim that ‘existence is an i’tibar ‘aqli – that is, a construct of the intellect’s activity on the quiddities it conceptualizes, and not some real feature of the concrete external object itself from which said quiddity is abstracted. Perhaps the Shaykh al-Ishrāq held something like this view.
The argument against the above, from the Kitāb al-Mashā’ir III.2.22, is as follows (my translation):
.فلو لم يكن للوجود حقيقة إلا مجرّد تحصّل الماهية, لم يكن حينئذ فرق بين الخارج والذهن, وهو محال
“If wujud didn’t have a reality other than the mere instantiation of a quiddity, then there wouldn’t be any difference between the extra-mental and the mental, which is absurd.”
Ta’liqa (تعليقة): a quiddity is univocally instantiated both in the mind and in external reality. But, the one has āthār (effects) which the other does not. Example: the quiddity fire, when instantiated in external reality, is hot, when instantiated in the mind, is not. Therefore, existence can’t be the mere instantiation of quiddity.