If you have the essence, you have it all. Or, not.

I came across an interesting argument by Frederick Copleston in his A History of Philosophy: Volume 1. I will give the quote in question and then share some comments.

Here is the quote: "Sextus Empiricus (c. A.D. 250), who is our main source for the details of Sceptic doctrine, argued against the possibility of proving any conclusion syllogistically. The major premiss-for instance, "All men are mortal"-can be proved only by a complete induction. But the complete induction involves a knowledge of the conclusion-"Socrates is a mortal." For we cannot say, that all men are mortal unless we already know that Socrates is mortal. The Syllogism is, therefore, an instance of a vicious circle. (We may note that this objection against the syllogism, which was revived by John Stuart Mill in the nineteenth century, would only be valid if the Aristotelian doctrine of the specific essence were rejected in favour of Nominalism. It is in virtue of our perception of the essence or universal nature of man that we are entitled to assert that all men are mortal and not because we lay claim to any perfect and complete enumeration of particulars through actual observation, which in the case in point would be out of the question...)" (pg. 188).

First, this is an interesting, and very early, critique of the syllogism. It reminds me that philosophy continues to repeat itself. Second, at first blush, Copleston's response makes sense. If we know the essence of the human, then a syllogism concerning that essence is not in need of a complete induction. Assuming mortality pertains to the specific essence of the human, then we can assume the same for all humans. The argument that a complete induction is needed is false, due to the reasons that Copleston gives.

However, is mortality essential to humanity? If memory serves me, for Aristotle, the nature of humanity is to be a rational animal. I don't remember him arguing that mortality is essential to or the essence of humanity (and I'm not going to take the time to look because there's something else to consider).

Even if I am wrong on that account, it still seems that Copleston's argument only works in relation to the essence of a thing. Take some aspect of humanity that is not essential, and yet all humans share. In that case, his argument fails and the critique stands that a complete induction is needed. Let's assume a syllogism that argues that all humans live on earth. And, since Socrates is human, Socrates lives on earth. Clearly, it is not essential to humans that they live on earth (one day some humans may live on the moon, for all we know). Even though it is surely true that that all humans live on earth, it is not clear that this is essential to humanity. In this case, it does not seem that Copleston's argument holds. In order to know the truth of the syllogism, we will need a complete induction of where all humans live. So, even if you have the essence, you still don't have it all.

Note:
Frederick Copleston, A History of Philosophy: Volume 1, Image Books-Doubleday, 1962.