He took Kant’s transcendental idealism as the foundation of his philosophy. He described transcendental idealism as the distinction between “phenomenon” (an object as seen by the senses) and the “thing in itself” (which Kant described as “noumenon”, or what exists independently of the senses).
He says that the objective world does not belong to the true being of things in of themselves, but is mere phenomena that exists due to the forms that lie “a priori” in the human intellect.
While Kant claimed that the noumena cannot be known by the senses, and that all knowledge of the phenomenal world is arranged by our mind, Schopenhauer pointed out that this is not applicable to our own bodies. So in that sense, since the body, specifically the inner aspect of our bodies are known to us through non-sensuous means, he said such is the way in which the true nature of all things should be known - and he called that the will. To him, every movement of the body is an expression of that will. Though we call it actions of the will, he says it is not the action of the will, but the instantaneous manifestation of the will that causes the movement. He says the same applies to inanimate objects, as matter transmutes into energy for their movement. But what he calls will is not the conscious actions of humans, as stated earlier, but the driving force of every movement.
And while Kant claimed that the noumena cannot be directly known by human knowledge which only applies to the phenomena, and Schopenhauer agreed with that too, he tried to best describe the will through his analysis.
In his work, The World as Will and Representation, he identifies the inner essence of everything as a “will” - blind and unconscious, outside space and time, and free from multiplicity. And he said that the world of objects in space and time, and related by causality, only exists as a representation dependent on the thinking subject, and not as a world in of itself. In other words, the “world” is an objectification of the will.
He said that a reader would be best prepared to understand The World of Will and Representation if they were familiar with “the divine Plato”, and even more so if they were familiar with the philosophy of the Upanishad-s of the Indian Veda-s. He considered Platonic “forms” to be existing on an intermediate ontological level between the will and the representation.
Other Philosophers
Nietzche said: “I belong to those readers of Schopenhauer who know perfectly well, after they have turned the first page, that they will read all the others, and listen to every word that he has spoken”
Charles Darwin quoted The World as Will and Representation in the Descent of Man. One can find in it ideas such as sexual instinct being the tool of nature to provide a quality offspring.