Imagine a single speck of light in the midst of impenetrable darkness. Imagine that this speck of light has multiple choices: to move in any direction it desires. Now, doing so in any fashion will result in said speck of light bumping into another speck of light after some time. Upon this happening, the universe of choices essentially explodes for both specks: not only can they just move about however they desire; they now also have a multitude of choices regarding the other speck they just encountered. So many things they can do, so many results.
Now imagine a galaxy, nay, a universe filled with such specks of light, all with their own unimaginable number of choices. Do I greet the other speck, hug it, hit it, try to start a relationship, become friends, procreate to create new tiny specks of light, convince other specks to engage in violent conflict with other specks or other erratic behaviour for reasons unbeknownst, and in how far does it matter whether it's a logical, well-reasoned decision?
This, in a galactic nutshell, is humanism. It's literally choices all the way down from here.
Now, voices will most decidedly be raised in protest that humans aren't simplistic enough to be reduced to mere specks of light, albeit with a level of sentience. Here is where behaviour experts (animal and/or human) and psychologists meet common ground: little of human behaviour is founded in intellectualism, and most of it is of the complexity displayed by even basic multi-celled organisms, whether in possession of a central nervous system or not.
In the end the uncaring universe presents all of us organisms with choices, from which we get to pick. By playing it just right we can survive, gain the upper hand within a group, obtain breeding access to that most attractive baboon female, become a famous historical leader, or pick our nose while the others aren't watching.
Okay, so we humans then are apparently as organized as a bunch of single-celled organisms bumping about in a jar filled with mucky water?
Not too far from the truth, I reckon. True, certain patterns can be detected, but something like the organized movements displayed by certain groups of fish or birds are scarce. Only major sporting events and riots - in as far as they can be seen as separate events - seem to evoke any kind of organization within a group of humans. Beyond this the choices made every few milliseconds by every waking human individual seem to result in a system which even defies chaos theory, moving into something far more surreal.
Imagine a universe filled with specks of light where each speck cared for no more than itself and those part of arbitrary categories such as 'mate', 'offspring', 'friends' and 'today's rising fluffy pop star'. All choices made by every speck would in the first place serve the speck itself, secondly those in the most important categories, thirdly by those a ring down, and so on. In the resulting chaos and mayhem all specks are rapidly killed, incapacitated, driven to suicide or watching re-runs of 'As the World Turns'. All ends with a single lonely speck of light drifting around for a while until finally blinking out.
This is the second part of humanism: to prevent this scenario from happening. Those who care about others without being able to fully justify this, except that they know that it will improve the lives of others. Those who run homeless shelters. Those who labour to right injustice done to individuals or groups. Those who save that kitten found dying in a rainy park. Those are humanists in at least part of their being.
People who care, who fight to right things. People who want that their choices are beneficial to all where possible. It's a choice to do so, every few milliseconds again. It's easier to be selfish. Less trouble. Less responsibility. Yet being a human being should come with the responsibility to be a humanist; to make the right choices for all.
Let everyone of us be the brightest speck of light in the universe :)