Purpose is a human-dependent measure. This is wrong-minded. There is no fundamental difference between the evolving form of a river, the minerals in a rock, or the phenotype of a species. They are all equally 'design-like'. It is simply a matter of shifting perspective and what you define to be the 'entity', and as a result, what is left for the 'environment'.Natural selection explains the design-like nature of organisms and their characteristics, without any need to talk about final causes. On the other hand, no natural selection lies behind mountains and rivers and whole planets. They are not design-like. That is why teleological talk is inappropriate, and why the Gaia hypothesis is so criticised. And overall that is why biology is just as good a science as physics and chemistry.
But his point is that there is no fundamental difference between the evolution of these phenomena. That is why he stated that there was "a Newton for the blade of grass". In order to accurately understand the evolution of life we can't project purpose into organisms. We can understand function. But not purpose.
But if you understand the above, then the question of purpose becomes meaningless, as you must realize it is a human-dependent description. Otherwise, just as you must ask "Does life have a purpose?" you must ask "Do mountains have a purpose?" or "Does the Universe have a purpose?" If there is a Newton for a blade of grass, and then you ask whether there is a purpose for the blade of grass, then you must ask about the purpose of gravity.
Not sure if I'm following here or if we are trying to say similar things. The quote re: a "Newton for a blade of grass" is intended to make the question of purpose irrelevant. Newton's theory of gravitational explained physical systems without purpose, just as Darwin's theory of evolution explained biological systems without purpose. They make any questions re: purpose irrelevant. All you can do is create internal subjective purpose for yourself. Not project a larger purpose into things that are inherently purposeless.
I think that's largely the case, but saying that "no natural selection lies behind mountains and whole planets" is not correct because natural selection doesn't lie behind life any more than mountains and planets. That's what bothers me here. Natural selection is a narrative, not a real thing. There is no actual 'selective force' or 'alternate histories' that the narrative suggests. We could look at a mountain range and say "What if the weather was different? Would the mountain range have evolved differently? How did the mountain range adapt to the weather?" But that language isn't as useful in geology. But, it is just a matter of language. Finally, appreciation of the above means that this doesn't make sense: Does the mountain range take a direction? That question is as tricky as whether or not life does. I think Ruse ends up at almost the right place regarding purpose, but to some extent, I think he just moved the ball by hanging onto the validity of the teleology question.But historical teleology — the question of whether evolution itself takes a direction, in particular a progressive one, is a trickier problem, and I cannot say that there is yet, nor the prospect of there ever being, a satisfactory answer.