You're welcome, Toby; thanks in return for yours. Summary: A) answering about the feedback I seek; B) defending premise P2, supreme value on reason, and explaining how to attack it; C) defending principle M2, maximum of personal freedom; D) defending rational discourse as the horse to pull the cart; E) sampling what can and cannot be reasonably agreed; and F) planning my next essay draft.
A. Robert says I "cover too much ground" to get my points across in the space available (Apr 30). I agree. That's a formal fault. If I could ask for more, I'd ask someone (despite the difficulty) to identify a content fault, i.e. one that invalidates the thesis, such as a principle that's unsupported in theory, or a practice that's infeasible. Or give the thesis a good denting in the attempt. Or reveal something new and interesting that's hidden to me.
But these are tall orders, given how the thesis is overcompressed.
B. No (to answer), I don't mean to imply in premise P2 (reason as a supreme value) that life itself has a purpose in reason.
Yes, I agree we may value something despite it being incapable of reason. In P2, I don't mean to imply that the value on reason is exclusive of other values; we'll still have a great number of other values. The theorist might even try to deduce from that great number the supreme value of reason, as reason recognizes the value of things. Not knowing the value of something, we're in greater danger of losing it, or of failing to create it in the first place (artificial things). See also my answer (E) to Mark's May 3 post.
For any who doubt the strength of premise P2 and wish to attack it: imagine that reason is lost from the universe leaving us behind mutatis mutandis. Now explain how we'd get along and ultimately recover unharmed. Then I'll agree that reason isn't supreme after all.
Or identify some other value V whose loss from the universe implies the loss of reason itself. Then I'll agree that V is supreme above reason.
Or identify some other value W whose loss from the universe we could not amend even while reason remained with us. Then I'll agree that W is co-supreme with reason.
C. You say, "Just as we don't know what actions will lead us to success (interplanetary [should be interstellar] colonisation), we might not know what actions will lead us to failure (extinction)." Here you imply a symmetry that would neutralize the utility of a maximized personal freedom (M2). But I believe that symmetry is already broken by M2. Consider: "if a given action does not reduce anyone's freedom to act, then it can hardly reduce the likelihood of eventual success" (p. 2). By the same token, it can hardly increase the likelihood of eventual failure. A maximum of personal freedom "compatible with equal freedoms for all" is more likely to avoid extinction than to cause it. This is what breaks the symmetry you imply. So M2 still holds (by prudence) as a means to M0.
D. I think the horse (to adapt your metaphor) already competent to pull. See figure F6. The introduction of guideways (attachment of harness) would enable the rational discourse of the public sphere on the left (strength of horse) to pull the decision systems on the right (cart). Currently we see the horse off chewing weeds instead of pulling the cart, or otherwise demonstrating its strength. But where you attribute this to the beast's incompetence, I attribute it to its being detached from the cart.
E. "If everyone is rational and working from the same knowledge and understanding, what would anyone disagree on?" I think the best answer (if I understand) is the obvious one: we might reasonably disagree about those things it would be reasonable to disagree about, such as favourite flavours of ice cream (trivial example), or certain aspects of the future (less trivial perhaps) that make no sense to agree about. But there's at least one non-trivial aspect of the future (I argue) that we cannot reasonably disagree about, which is also a timeless constant, and therefore a destination to steer for. This is M0, from which I deduce a theory and means of future steering (aka morality).
F. In my next draft (which I'm planning now), I'll try to fix the formal fault that Robert has identified by giving the text a lot more room to breathe, and letting it answer itself the questions you and others pose. I'm grateful to you all on this account because I've generally no access to critical readers.
Mike