It's nothing to do with analyticity; this is another point entirely. Feel free to choose an example that does not involve analyticity. What I am saying is that when faced with the contradictory statements "no tree is a plant" and "this thing (demonstrates) is a tree, it is also a plant," it is not a legitimate thing to say that of the two statements we can keep whichever we want. We are obliged to keep the second and deny the first. The person who says "oh, so it cannot have been a tree if it is a plant" is doing the opposite. Why are we obliged to deny the first statement and keep the second? Quite simply because empirical evidence supports the second and this denies the second. (By "empirical evidence" I mean evidence about how we talk. I.e. we really do call this a tree and we really do call this a plant).Sure, you could deny that the empirical evidence supports the claim that if we think X is a tree then we will tend to think X is a plant. However, were you to do so then you would be incorrect. Similarly you cannot deny that some philosophy is not saddening because you will find many things that are philosophy/philosophical activity/whatever that are not saddening. The counter-claim that these things are not therefore philosophy is just manifestly false. We just do call these things "philosophy" and we just do think of these things as "philosophy."
Were you to respond that "sure, we call them philosophy but that doesn't make them philosophy" I am afraid that I have lost you. What are trees other than those things that we call a "tree" and what is a philosophical activity other than those things that we call a "philosophical activity"? Since we do call non-saddening things "philosophical activities" (and therefore there are non-saddening things that are philosophy activities, which is the same thing) Deleuze is wrong.I do take Andrew B's point that such writings are often to be taken with a pinch of salt (as the author perhaps intends), and if Deleuze's intention is not to categorically claim that all philosophy really , actually, is saddening then maybe we are wasting our time to some degree. Nevertheless once we do take such writing with a pinch of salt we don't really have anything hard to grasp onto in terms of what the author is trying to say. Perhaps the author is therefore failing to say anything. Perhaps I am a chauvinist, but any philosopher (or anyone writing on philosophical topics) should avoid any language that is poetic, flowery, metaphorical or requires taking with a pinch of salt since I have yet to see a single example where this actually helps elucidate something rather than add another veil of confusion behind which the author can hide from actually dealing with the subject at hand.*M.*Yes I know I have criticised someone for using a metaphor by using a metaphor. But it's OK because I'm not philosophising but ranting.