In a theory of the “practical understanding” Brunner develops a doctrine of motion that parallels Einstein’s theory of relativity and Husserl’s phenomenology (thing-concept, perception of space and time, motion-concept). Brunner’s doctrine emerged, however, independently of both these conceptual frameworks, and stands in the broadest sense in the context of discussions in late 19th century philosophy regarding psychology, physiology and physics.
Derived from the doctrine of motion, Brunner’s psychology denies the possibility of a definitive "understanding" of the world; and he arrives, like Spinoza, at body-soul identity. His philosophy thus stands against the Enlightenment’s ideology of reason, as well as against Kant’s dualism. In their place, Brunner posits instead the unity of psychic movement, experienced sequentially as feeling, knowing and willing; and which is made manifest in the form self-interested purposiveness. The practical intellect, which deals with all concrete, quotidian thinking up to and including scientific thinking, has significance solely in terms of biological survival (to be sure, this is not in the sense of a genetics-based epistemology: Brunner leans heavily away from Darwin’s theory of evolution). The practical intellect serves solely the furtherance of our interests.
This point marks the transition to the practical philosophy, to the doctrine of society and the state, to which Brunner dedicated just as much effort. It is derived theoretically from his psychology, and elaborated through the example of hatred of Jews. Brunner identifies the psychological mechanisms through which all people, to a greater or lesser degree, act on the basis self-interested thinking. This approach, developed in the theory of hate elaborated in Brunner’s books on the Jewish question, clarifies not only the psychological basis of antisemitism, but also of every form of social exclusion of "the other.”
In his theory of the democratic state, Brunner argues for the recognition of dissenters and outsiders as equal people. The goal remains individual freedom, but this must be tempered by the state: Brunner rejects Anarchism. He designates the egalitarianism and collectivism flowing from communism as exactly the same type of "freedom strangler" as fascism. The satisfaction of economic needs is certainly an important goal, but private possession is a fundamental interest of mankind, and must be taken into account, just like the prestige interest (the need for honour and human vanity) and the love interest. Brunner considers as too one-sided Freud’s eros-based theory.
Brunner sees the actual social and political relations in the state and in society as being established in general by people who absolutize (ie. idolize superstitiously) any and all practical interests. To be sure, this flows from natural egoism and its self-interested pursuit of love, possession and honor/vanity; but its excess, its absolutizing, is harmful, culminating in pride and greed, and must be checked through correct thinking.
Thinking and living are identical. One of the main principles of Brunner’s philosophy reads: “What you think incorrectly, you must live wrongly.” Correct thinking means aligning thought with reality, rather than the reverse.
Standing as shining ideals are the intellectual achievements and living examples of the great “people of spirit” (Brunner names, among others, Spinoza, Christ, Beethoven, Rembrandt, Moses, Socrates), who do not absolutize their personal interests, but rather think and live within the realm of "absolute truth." In them, the spiritual is not utopia, but rather the real present. "Absolute truth" is for Brunner the thinking of the One: the not relative, not metaphysical Absolute, Spinoza’s Deus sive Substantia. Reality has thus two dimensions: in essence it is the absolute, spiritual One (absolute idealism); in appearance it is the relative many motional things (relative materialism). Both dimensions of reality are apprehended through corresponding "faculties" of human thought. With “spiritual thinking," the person grasps the absolute One as artistic or philosophical thinking or as mystical love. With the "practical understanding," people grasp the relative many as what they feel, know and want. Along with both of these faculties, Brunner speaks of a third faculty of thought: "superstition," or the “Analogon,” in which the relative is conceived not as relative, but rather as absolute, and which is expressed as religious, metaphysical and moralistic thought. Whosoever thinks in this third way, Brunner designates as "Volk."
In actual life a person’s thought is either thoroughly spiritual, or thoroughly superstitious-folkish –, that is what Brunner maintains, and thus he names his philosophy a "doctrine of the people of spirit (Geistigen) and of the Volk". Brunner does not believe in a historical progress toward spiritual thought, and he criticizes Hegel on this point. Nevertheless he rejects also the pessimism of Schopenhauer. Pessimism and optimism are moments of the practical life experience; they do not belong in philosophical thinking. Joie de vivre is the goal of philosophical life formation (Brunner follows here Spinoza as a kind of Epicurean). This state of happiness is attainable only by the few who are ready to give existence to their spiritual essence. The "Volk" does not arrive there because it distorts truth again and again into superstition, and thereby thwarts its own wellbeing. Character is fundamentally unchangeable. People cannot be improved: only political and social relations can and must be improved.
Brunner’s philosophy is directed on the one hand toward improvement in political and social relations, and on the other toward individual self-improvement, a life-changing alteration in the direction of the spiritual. Brunner saw academic philosophy as having unfulfilled the demand that philosophy must not be merely theoretically engaging, but also practically activating. And so his doctrine is at once a battle against phony depth and phony thinking, and against an ultimately merely instructive, but not life-changing, general formation, as in aestheticism. (Here there are some parallels to Nietzsche, whose skepticism Brunner however sharply attacks).
Brunner aimed at a simple system in his philosophy with a minimal number of terminologically-fixed concepts.
At first, Brunner wanted to write only one book: Die Lehre von den Geistigen und vom Volke (The Doctrine of the people of spirit and of the Volk), divided into three volumes: Practical understanding, Spirit and Superstition (Analogon). There appeared in this systematic form only the first volume (in two half-volumes). It appeared under the title of the entire programme: Die Lehre von den Geistigen und vom Volke. A lengthy “Announcement” at the beginning of this work alludes to the entirety of the doctrine, but only the theory of the practical understanding is treated fully here.
That Brunner did not write the other two volumes under the pre-formulated titles “Spirit” and “Analogon” does not mean that he neglected to treat these topics in detail. In fact, the opposition of these two modes of thought is effectively the central theme of all Brunner’s works, even in the book on the practical understanding. The reason for this is that, according to Brunner, these three concepts designate the three—and only these three are possible—modes of thought (or, as Brunner calls them, “faculties”). The first of these, the practical understanding, is found in all people. The other two, Spirit and the Analogon, are never found together; rather, the practical ,understanding is always conjoined with one or the other of these perceptual frameworks, ie. either with Spirit or with the Analogon.
This conjointedness in mankind, whether factual or not, Brunner designates under the concepts "people of spirit" (“Geistige”) and "Volk." Whoever thinks the practical understanding on the basis of spirit is of the spirit; whereas those who think the practical understanding on the basis of the Analogon are of the Volk. And so we have in the first place a doctrine of faculties that seeks to encompass all possible perceptual, cognitive and existential modes; and in the second place, an anthropological doctrine of the spirit and the Volk that forms for Brunner the explanatory basis for all actual individual, social, political and historical conditions.
In his doctrine of the faculties, Brunner distinguishes three manifestations within each of the three faculties. In the practical understanding the psychological "specificates" are feeling, knowing and willing. In spiritual thinking these are modified into, respectively, art, philosophy and mystical love. In analogical/superstitious thinking they become, respectively again, religion, metaphysics and moralism. These nine are the categories in which all thinking and feeling, each type of perception and speculation, each mode of being, all consciousness, can be inserted.