Art is anything evolved in our social species to attract attention. This may be vague, but it’s actually the only definition that is general enough to cover everything we call art, from cave paintings to dadaism to flashmobs. How it manages to attract attention and what side effects it has is what makes all the difference. You may never publish and keep it secret, but it’s still art, even by this definition, just like masturbation is still part of human sexuality. It’s just that something has to first attract attention of its own creator, who then may or may not use it to attract attention of others.
Philosophy is part of art, so defined. An insight or a speculation that comes into your head first needs to attract your attention, to seem sufficiently new and interesting to you. Only then you may share it with others who will assess its interestingness for themselves.
What sets philosophy apart, then? I think it’s in how it attracts attention, in what it appeals to in order to command attention. Philosophy was the first art to feed upon, and in turn stimulate, human curiosity for how things really are, the first one that appeared after humans could discern that “what really is” is not always what it looks like. That set philosophy apart from other forms of art that make no truth claims; some time later it also parted ways with religion (which also has its roots in generic art) in that philosophy, further, made use of the capacity of reasoning. But the evolutionary basics remained the same: whatever attracts attention, survives. The integral estimate of survivability, for philosophy, is its persuasiveness: if I can follow the reasoning and it rings true to me, it is good philosophy; if, in addition to that, it is also new and interesting (“gosh, how come I didn’t think of this before!”), it is excellent philosophy that will survive (at least) in my mind and procreate by affecting (at least) my own philosophizing.
Further down the road, however, an interesting thing happened. The persuasiveness selector was continuously refined and made stricter, and at some point it turned into something quite different: verifiability. Science was born, and it gradually separated from philosophy which continued to use the old persuasiveness selector. (Verifiability, in turn, split into internal (mathematics) and external (the rest of sciences)).
So, what can we make of progress in philosophy, in this view? Does it even make sense? Unlike scientists, philosophers never achieve a “widespread agreement” on anything. But neither can we appeal to the sheer volume or diversity of the modern philosophy – if “every man is his own philosophy” then the only progress we have is demographic.
As we have an evolutionary process here, I would propose a purely self-referential evolutionary definition of “progress”: it’s when the descendants are better adapted to a wider set of conditions than were their ancestors. Namely, if both these points are true:
(1) best “old” philosophy completely or partly fails the modern persuasiveness standards: sounds arbitrary, contradicts itself, does not reason strongly enough, misses obvious objections;
(2) best “new” philosophy adequately answers both modern and old persuasiveness standards,
then we can conclude we have genuine progress in philosophy.
On whether these conditions are valid, or on whether we can in fact determine their validity, I won’t speculate. The only thing I would like to point out is this. One critical difference between persuasiveness and verifiability is that the former depends on simplicity while the latter does not. Verifying the existence of the Higgs boson is extremely complex; if you write down the entire procedure it would likely take hundreds of volumes. Despite that, this verification is accepted by scientists, and hailed as a major achievement. In philosophy, if you need to write hundreds of volumes to prove your point, you can give it up at once: it will never fly. With this in mind, it should be clear how extremely difficult it is to make any progress in philosophy: there’s only a finite number of simple ideas, and quite naturally many of them were exhausted by the old philosophers. New philosophers, in order to produce more philosophy, have no choice but to go for more complex concepts and/or reasoning, and more complex automatically means less persuasive.
P.S. I’m aware my ideas here are reminiscent of Dawkins’ memes but those carry some extra baggage I would like to avoid here.