What's Philosophy and Paul Graham
I find it LOL.
Formal logic has some subject matter. I took several classes in logic. I don't know if I learned anything from them. 
 In practice formal logic is not much use, because despite some progress in the last 150 years we're still only able to formalize a small percentage of statements. We may never do that much better, for the same reason 1980s-style "knowledge representation" could never have worked; many statements may have no representation more concise than a huge, analog brain state.
Eh?? What formal logic is he talking about? Taken at face value, formal logic is synonymous to symbolic logic, mathematical logic, or logic by formal languages. We thank formal logic for computer algebra systems such as Mathematica, and much computer language advancements such as lisp, OCaml, Haskell, and automated math proof systems or assistants such as HOL. (See: The Codification of Mathematics; State of Theorem Proving Systems 2008.)
So, clearly he doesn't mean this. Perhaps he means Syllogism or propositional logic. You know, basically reasoning by words, in classical philosophy. But without these, formal logic as we know it, including so-called “boolean algebra”, fundamental to computers, wouldn't have developed.
What is Philosophy?
The whole essay is quite valueless to me. Let me tell you what philosophy is, its essence: Philosophy is THINKING.
Thinking, when done in a serious way on a subject matter, that you do hard, think, think, think, keep thinking, then write down your thoughts, and think more, think about your thoughts, try corrections, and observe and learn and think more, doing this for months or years, until, you might have something to say about a subject, even if the conclusion is “i dunno”, then, it is philosophy.
Philosophy, is the root of sciences. In the beginning, science, art, poetry, religion, philosophy, are all kinda mixed up together as one subject. Then, they gradually diversified. In particular, philosophy turned into today's cosmology, astronomy, medicine, geometry, chemistry, physics, math, linguistics, biology… More concretely, when some question is resolved by what we call scientific methods today (for example: observation, experiment, analysis, prediction), they are not called philosophy anymore, but science. But, if you answer the question by basically just thinking, or thinking with other methods that we don't have a name for yet, that's philosophy!
Paul's essay, the elaborate 4.7k words, is the epitome of philosophy! You can see, that he is thinking, thinking, and the subject of his thinking is “philosophy”, or “how to do philosophy” as his title suggests. And, what's his method? Did he go by the science of statistics? social experiments? Systematic analysis of literature? Archeology of examing cultural artifacts? CAT-scan of philosopher's brains? No. His method, is just thinking. Thus, he's doing philosophy right there.
However, in his essay he thinks that philosophy is useless. Without his interest and college major in philosophy, he wouldn't be what he is here today, and may not be as successful, and of course wouldn't write this essay about philosophy.
Philosophy, or call it thinking seriously, and in watered down form is called “critical thinking” in today's universities, teaches and trains you one thing: think! And that, is fundamental to any intellectual pursuits, even if you end up not understanding the ancient philosophers and its history, such as Plato, Aristotle, that Paul cites, or classical subjects such as free will, mind and body problem, cosmology, ethics, esthetics, epistomology, ontology, or the existence of Allah!
The culminating point of Paul's essay is this proposal:
… Instead of trying to answer the question:
What are the most general truths?
let's try to answer the question
Of all the useful things we can say, which are the most general?
I think its quite silly. He characterized philosophy wrongly and narrowly, by linking it to just metaphysics, and somehow, seems to have a fetish towards Aristotle.
Aristotle is a dead figure, and so is metaphysics. I doubt that these subjects are more than 1% of the subject matters of journals and publications among today's philosophers.
PS: John Wiegley (author of Emacs's eshell, among other things), has also written a essay responding to Paul's article: [• Response to PG's “How to Do Philosophy” By John Wiegley. At newartisans.com , Accessed on 2010-10-10 ]
The article address directly on Paul's proposal on usefulness. I haven't read Wiegley's writing before, am very impressed. Excellent article.