A “Humanist” is someone whose world-view gives special importance to human concerns, values and dignity. If that is what a Humanist is, then most of us qualify as Humanists. But those who organize under the label “Humanism” tend to sign up to a narrower view.
Accepts the methods of science
A key humanist value is the high importance set on scientific method and reason as the proven route to secure knowledge about the universe. Humanists understand that science is a method, not a set of facts. Scientific method is the best way of finding reliable knowledge.
Rejects transcendental realm
Many Humanists narrow their view further to include Naturalism. Naturalism is the belief that nothing exists beyond the natural world. Naturalism can be similar in certain ways to Materialism or Physicalism that finds insufficient evidence and reasons to accept the existence of a non-material transcendental realm. This distinguishes them from the religious who believe in a celestial realm of existence.
Is it possible to practice the methods of science and still reject the existence of a transcendental realm?
Transcendental realm of concepts
There can be no science without entering the transcendental realm of pure concepts. Whatever theory a scientist proposes, it must be explained in clear conceptual form so it can be rethought by others. Communication between scientists is only possible because the universal concept of a triangle that my mind grasps is the same as the concept which someone else's mind grasps.
To explain the science in a billiard shot we, we find the right concepts and add a second process in the conceptual realm.
"The purpose of my reflection is to construct concepts of the process. I connect the concept of an elastic ball with certain other concepts of mechanics, and consider the special circumstances which obtain in the instance in question. I try, in other words, to add to the process which takes place without any interference, a second process which takes place in the conceptual sphere." POF 3.0
The universal concept of triangle is not derived from the material world because it is not sense-perceptible. It is an ideal that contains all possible triangles. We can find and observe particular triangles in the world but not the universal concept triangle. The concept triangle is found in our mind as part of the transcendental realm of pure concepts.
Not everything in our mind is a pure concept. We have to make an effort to rise to the level of pure concepts just as the scientist, mathematician and philosopher do who must think in pure concepts to accomplish anything. Many mathematicians believe a non-natural mathematical realm exists. They suppose that 2 + 2 = 4 is made true by how things stand in this universal, non-natural, mathematical reality. If such a mathematical reality exists, then the limitations of naturalism are false.
Does this make reality dualistic?
Does the existence of a transcendental world in addition to the sense-perceptible world mean reality is dualistic? Not at all. Mathematics is expressed in the world and it also exists in the realm of pure concepts. This is true of other concepts. We experience the full reality from two sides, an observable part in the world and a conceptual part that arises in our mind.
"It is due, as we have seen, to our organization that the full totality of reality, including our own selves as subjects, appears at first as a duality. Knowledge transcends this duality by fusing the two elements of reality, the percept and the concept, into the complete thing." POF 7.0
The concept is the unifying rule or principle that unites the separate parts we perceive in the world.
“All attempts to discover any other principle of unity in the world than this internally coherent ideal content, which we gain for ourselves by the conceptual analysis of our percepts, are bound to fail. Neither a personal God, nor force, nor matter, nor the blind will (of Schopenhauer and Hartmann), can be accepted by us as the universal principle of unity in the world.” POF 5.9
To remain true to the principles of Humanism such as scientific inquiry requires the acceptance of a transcendental realm.