Heisenberg's uncertainty principle, now there's an interesting concept. We can never know the precise location and momentum of a particle simultaneously. It might lead one to believe that all experience is somehow relative, imprecise and illusory. Experience is relative to the observer? Well, that might work for opinions but what about our experience of the world. I mean we can only experience the world as it is regardless of our ability, or inability, to understand and describe reality. The uncertainty principle, or more correctly the indeterminacy principle, describes things as they seem to be at the subatomic level but does that say anything about our macro level experience? Schrodinger designed a thought experiment to illustrate the absurdity of the notion that reality is somehow ultimately determined by observation (Schrodinger's Cat). The cat is either dead or alive independent of our observation. Follow the links for these guys to in depth explanations of the consequences of their theories.
What does any of this have to do with Humanism and its ethic? Well that depends. What is the/an ethic of Humanism? For me it is all about creating the meaning/purpose of life, both for the individual and collectively. Existence doesn't come prepackaged with certainty of meaning. So since I believe this life is all we have I also believe it is imperative that we decide and act upon an explanation for it's purpose. Furthermore I believe it is not enough, not a significant enough principle, for each individual to embark upon a course, to choose a meaning in isolation from the rest of humanity. I would argue that connectedness with other humans is as close as we can come to a "spiritual experience". In fact for me it suffices as an explanation for what some call the spiritual nature of life. Of course I would replace the word 'spiritual' with 'human'. Perhaps I can coin a whole new concept and call it "humanal"? Well maybe not.
Okay, my point is that the range of human experience, the good the beautiful the bad and the ugly, is what life is all about. And what we can do to make that experience the most meaningful is tied up with our success in maximizing real connections with our fellow humans. I mean life is all there is, it is the ground we all share and I believe it is what matters in this universe. How to go about sharing that ground is elementally important, to me. The spectrum of emotions and other physical experiences of our bodies (pain, pleasure, hunger,satiation) are at the same time unique and universal. As individuals we tend to prefer maximizing the good and the beautiful. As humans we could/should seek to maximize this tendency by not only respecting the rights of everyone to experience the good and the beautiful but by actively enabling each and every person's (and perhaps extend this to all sentient creatures) access to the means of achieving this end. I believe when you take any action to relieve another's pain, suffering, hunger, thirst, lack of freedom you acknowledge your belief in that person's ultimate value and your own connectedness to their humanness.
So while our best theory of everything (quantum mechanics) describes reality as indeterminate at its most basic level and yet our everyday experience is as certain as a dead cat we are left with the everyday problem of determining what counts as a meaningful life and finding ways to be as concretely connected as possible to the reality that is life.