I've just finish reading an interesting little book by John Sheriff on this topic. The riddle in question is that of the Sphinx, (as described by the sometimes inspiring and often muddle-headed Ralph Waldo Emerson) and it seems C. S. Peirce had hopes of publishing a book to be called A Guess at the Riddle which Sheriff believes was to set forth Peirce's unified theory of cosmology, including the meaning and purpose of human life. Sheriff takes a shot at describing that theory.
Peirce was a remarkable thinker, though difficult at times to read and understand. I find him fascinating, and if Sheriff's interpretation is accurate, I find this "unified theory" fascinating as well. Peirce apparently speculates (this is a very rough and simple attempt at description after a first read) that the universe could have resulted from a nothingness which was somehow pregnant with potential, that time came into being when it somehow became potential, that it slowly came to take on characteristics of what we perceive as order by "habit", a kind of repetition of possibilities, that the repetition by repetition came to be in the nature of rules or law, that the universe over the ages came to be more complicated but also more organized, that its tendency toward organization is something we participate in and is a kind of creative process in which we can play a part. We do so, and should do so, through the exercise of Reason; however, we understand that Reason is appropriate through sentiment. Also, we function as a community in the exercise of Reason. Reason, and knowledge, is social. Reason and ethics derive from knowledge which has developed over thousands if not millions of years as we humans have interacted with each other and the rest of the universe, and found through trial and error--and sometimes even through the use of the experimental method--that certain acts had certain consequences, and were desirable to us as a community. The exercise of reason and enjoyment of knowledge being social, we ourselves should be social, selfless, creatures--loving, apparently in the Christian sense--in order to assist and participate in the process of creation.
Peirce evidently came to such conclusions through his creation (he would say discovery, I think) of a triadic system inherent to all which he felt explains everything, which he factored into all of his thought and work.
Part of what I find interesting is the fact that (at least to someone like me, with no knowledge of physics) it sounds familiar to what one hears may be a plausible explanation for the development of the universe. Peirce's cosmology, like his pragmatism, was evolutionary, of course; the influence of Darwin on the early pragmatists was profound.
I'm also struck by the fact that his conclusions regarding Reason and morality, and love, arise from nothing that is supernatural, or beyond nature, but from the very development of the universe itself. He doesn't vary from his fondness for logic and the experimental method in making these inferences, though he sees those methods as themselves developing naturally as consequences of creation. Finally, it seems interesting that this theory lead him to conclusions regarding how to live which seem on their face to be very similar to the Stoic belief that a kind of Divine Reason governs the universe, and that we partake in that Reason when we live according to reason and nature, and leads to the position that we should be selfless and love others.