There are so many great passages it's hard to pick one. Here's one choice:
"Yes, sir," replied the count; "l have sought to make of the human race, taken in the mass, what you practice every day on individuals - a physiological study. I have believed it was much easier to descend from the whole to a part than to ascend from a part to the whole. It is an algebraic axiom, which makes us proceed from a known to an unknown quantity, and not from an unknown to a known; but sit down, sir, I beg of you."
Monte Cristo pointed to a chair, which the procureur was obliged to take the trouble to move forwards himself, while the count merely fell back into his own, on which he had been kneeling when M. Villefort entered. Thus the count was halfway turned towards his visitor, having his back towards the window, his elbow resting on the geographical chart which furnished the theme of conversation for the moment, - a conversation which assumed, as in the case of the interviews with Danglars and Morcerf, a turn analogous to the persons, if not to the situation. "Ah, you philosophize," replied Villefort, after a moment's silence, during which, like a wrestler who encounters a powerful opponent, he took breath; "well, sir, really, if, like you, I had nothing else to do, I should seek a more amusing occupation."
"Why, in truth, sir," was Monte Cristo's reply, "man is but an ugly caterpillar for him who studies him through a solar microscope; but you said, I think, that I had nothing else to do. Now, really, let me ask, sir, have you? - do you believe you have anything to do? or to speak in plain terms, do you really think that what you do deserve being called anything?"
(From Chapter 48, "Ideology")