One objection against the intellectus essentiaea argument of Aquinas for establishing the real distinction between the essence and existence of any qualitatively finite thing comes from John Buridan, and is based on the breakdown of the substitutivity of identicals. All that is verbiage. Let me explain.
Aquinas says one way we can know that essence and existence are really distinct among qualitatively finite things is because we can know their essence without knowing their act of existence. But if existence was essential to such things then this wouldn’t be the case.
As Klima puts the argument:
1. The nature of c is known.
2. The existence of c is not known.
3. Therefore, the nature of c is not the existence of c.
Or more formally in predicate logic:
1. Kn
2. ~Ke
3. e =/= n
Seems simple enough. But Buridan says the argument fails because there are instances where we can know something as A but not something as B even if it is both A and B.
To borrow from Feser in 5 Proofs: I can know that Aspirin is a pain reliever without knowing that aspirin is acetylsalicylic acid. So, while the concepts are distinct, that does not mean there is a real distinction between Aspirin and acetylsalicylic, because aspirin really is acetylsalicylic acid, after all. And if that is the case, then just because we can know A without knowing B, that does not show that A and B are really distinct.
This objection, however, can be overcome by understanding whether the concepts involved are logically independent, which is Feser’s defense, complementing a point previously made by Klima.
For if I knew all there is scientifically about aspirin — that is, to grasp its full quidditative definition — then I cannot fail to know that aspirin is acetylsalicylic acid, whereas even if I knew all there scientifically is about Aspirin, I could still fail to know that Aspirin qua Aspirin is the second best selling pain reliever on the market, because in the second case, the concepts are logically independent of one another, whereas in the former case they are not, and so a COMPLETE grasp of the essence of Aspirin would include the knowledge that Aspirin is acetylsalicylic acid, but not that it is a popular seller or that it is Bob’s preferred pain reliever or currently on sale at Walmart.
Further, to know that Aspirin is acetylsalicylic acid tells me something about ANY Aspirin that has existed, currently exists, or could exist: that it will always be acetylsalicylic acid. But to know that Aspirin currently exists does not tell me whether Aspirin has always existed or will always exist, and that is because the concepts are logically independent: because essence and existence are really distinct.