system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is the difference between Logic before Frege and after him?

In my first year of university, I visited a Logic seminar, which mostly taught propositionale calculus and first order logic but mainly in a way of just using it and not showing how it actually developed. Since then, I often read Frege practically reinvented logic in the 20th century. 
So my question is, what did he actually do, what was his contribution, and is all pre Fregian logic obsolete now?
gpt
The key difference is in the analysis of general statements like "Humans are animals." For Aristotle, the logical form of "Humans are animals" is relatively similar to the logical form of "Socrates is human." Both are predications---in the first, one's predicating animal of human and, in the second, one's predicating human of Socrates. For Frege, these two sentences have radically different logical forms. Whereas "Socrates is human" is a predicational sentence, the logical form of "Humans are animals" is actually that of a conditional, with a specific sort of logical operator that didn't really exist until Frege: a universal quantifier. In contemporary notation, we'd represent this: ∀x(Hx --> Ax). This says, "For all x, if x is a human, then x is an animal." So, on Frege's analysis, when one says "Humans are animals" one is really speaking of everything and saying if it's a human, then it's an animal.

This quantificational analysis of generality gives contemporary logic much more expressive power than ancient logic. For instance, we can express mathematical notions like a relation's being a dense ordering relation in contemporary logic, but there's no way to do this in Aristotelian logic. And it's not that Aristotelian logic simply needs an upgrade---the way of thinking about generality needs to be re-thought at the ground level.