Humanism & the Supernatural Metaphysics of Humanist Philosophy

As a general principle, humanists are not particularly concerned with anything that might qualify as “the supernatural.” Most humanists reject the existence of the supernatural entirely, but there is nothing self-contradictory about a person who is both a humanist and who believes in something that might qualify as supernatural.
What is a problem for humanism is not so much belief in something that is supernatural but the reliance upon the supernatural as an explanatory mechanism for the universe, for life, for existence, for humanity, for morality, etc. Such a reliance is rather common in human cultures generally and human religions specifically; the rejection of the supernatural as a means to explain anything, on the other hand, is an important characteristic of humanism.
One of the Affirmations of Humanism is:

”We deplore efforts to denigrate human intelligence, to seek to explain the world in supernatural terms, and to look outside nature for salvation.”

In a similar vein, the First Humanist Manifesto states:

”Humanism asserts that the nature of the universe depicted by modern science makes unacceptable any supernatural or cosmic guarantees of human values. Obviously humanism does not deny the possibility of realities as yet undiscovered, but it does insist that the way to determine the existence and value of any and all realities is by means of intelligent inquiry and by the assessment of their relations to human needs. Religion must formulate its hopes and plans in the light of the scientific spirit and method.”

The Second Humanist Manifesto appears to suggest that belief in the supernatural is incompatible with humanism:

”We find insufficient evidence for belief in the existence of a supernatural; it is either meaningless or irrelevant to the question of survival and fulfillment of the human race.”

Note, however, that the second half of the sentence seems to contradict the first. It is true that humanism regards supernatural claims either as meaningless or, if meaningful, then not relevant to human survival and fulfillment. However, being irrelevant does not mean being false. Thus, while a person might believe in something supernatural while still being a humanist (however uncommon that may be), a person cannot rely upon the supernatural to explain things about the universe and life and still be a humanist.
The reason why the supernatural is essentially irrelevant for humanism is that humanists are fundamentally naturalists. As naturalists, humanists rely upon nature and natural explanations to understand the universe and life. The universe is believed to have natural origins. Life on this planet has natural origins. Morality has natural origins. Nothing supernatural can or is needed to explain such things. As for the metaphysical question about the nature of reality, humanists answer: it is natural.
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What is Humanism? What is Humanist Philosophy?

At its most basic, humanism involves any concern with humans (including human needs, human desires, and human experiences) first and foremost. Often, this also translates into giving human beings a special place in the universe on account of their abilities and faculties.
Working from this general conception, we can see that humanism is not a particular philosophical system or a set of doctrines, or even a specific system of beliefs. Instead, humanism is better described as an attitude or perspective on life and humanity which in turn serves to influence actual philosophies and systems of beliefs.
The difficulty inherent in defining humanism is summed up in the Encyclopedia of Social Sciences' entry on Humanism:

"Humanism as a technical term and as an intellectual or moral conception has always leaned heavily on its etymology. That which is characteristically human, not supernatural, that which belongs to man and not to external nature, that which raises man to his greatest height or gives him, as man, his greatest satisfaction, is apt to be called humanism."

"Humanism thus means many things. It may be the reasonable balance of life that the early humanists discovered in the Greeks; it may be merely the study of the humanities or polite letters; it may be the freedom from religiosity and the vivid interest in all sides of life of a Queen Elizabeth or a Benjamin Franklin; it may be the responsiveness to all human passions of a Shakespeare or a Goethe; or it may be a philosophy of which man is the center and sanction."

Humanism can also be better understood when considered in the context of the attitudes or perspectives it is normally contrasted against. On the one hand is supernaturalism, descriptive of any belief system which stresses the importance of a supernatural, transcendent domain separate from the natural world in which we live. Quite often this sort of philosophy describes the supernatural as being more "real" or at least more "important" than the natural, and hence as something we should strive for — even if it means denying our needs, values, and experiences in the here and now.
On the other hand are types of scientism which take the naturalistic methodology of science so far as to deny genuine importance or, at times, even reality to human feelings, experiences, and values. Humanism is not opposed to naturalistic explanations of life and the universe — on the contrary, humanists see it as the only viable means of developing knowledge of our world. What humanism does oppose are the dehumanizing and depersonalizing tendencies of modern science.
It is one thing to observe that humans are not valued by the universe at large, but quite another to conclude that therefore humans are not really valuable after all. It is one thing to observe that humans are but a tiny aspect of the universe and even of life on our own planet, but quite another to conclude that humans can have no important role to play in how nature progresses in the future.
We can conclude then that a philosophy, world view, or system of beliefs is "humanistic" whenever it shows a primary or overriding concern with the needs and abilities of human beings. Its morality is based upon human nature and human experience. It values human life and our ability to enjoy our lives so long as we don't harm others in the process.
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