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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