The Will and the Id as Eros
The Will/Id in Schopenhauer/Freud (respectively) represent a sense of life. They think of it as pre-personal and connected to the social. Eros, the sexual impulse, for Freud, was the fundamental drive of the Id; this was, in turn, fundamental to the personality of the human being. It also represented the individual’s connection with the mass consciousness.
“Love, Heart and Sunset,” by B Axelle, taken from [PublicDomainPictures.Net]; this image is in the public domain.
Kantian Pre-Consciousness
Freud’s idea of the Id, while perhaps (or perhaps not) only coincidentally resembling Schopenhauer's will, like the will, is enabled by Kant’s work on epistemology. In Kant’s Critique of Pure Reason, he discusses knowledge as being enabled by so-called “apriori” (prior to experience) understanding, which is related to the experience of space (objective apriori) and time (subjective apriori). What made Kant’s idea so provocative was his introduction of the pre-consciousness. Descartes before him had talked about the consciousness as a thing in itself, separate from other objects in reality, but he did not address the mechanism of knowledge acquisition itself. In order to explain how sensory input becomes knowledge, Kant had to model a full-fledged pre-consciousness to assemble the manifold into perceived objects of being.
Kantian Ego Versus Id
Kant’s pre-consciousness seems to roughly correspond to Freud’s ego. According to Freud and Schopenhauer, the Id is the actual only real thing in itself. We ascribe being to objects by the Id, through the ego. This process takes place in the ego, which concretizes what Kant would call the manifold of experience.
The Id as the Essence of Man
The Id/Will, however, represents something which I do not believe Kant ever attempted to delve into. The Id/Will is beyond the confines of Kant’s apriori and is the originator of the pre-personal instincts. Additionally, the Will/Id is the repository of the mass consciousness. Understanding the essence of man as Will/Id is helpful in understanding Heidegger (and vice-versa). In Heideggerian terms, part of the human being connects to Dasein directly; that part is the Will/Id. Although Heidegger himself would not use the term, the appropriating events of Dasein have a kind of mass consciousness which appropriates man on the level of his essence. The Id is simultaneously the essence of man and the source of the essences which man projects onto Dasein in the clearing of being.
Eros and Thanatos
The Id, which is both the essence of man and the source of the essences of those objects which man draws as radically other from amidst the nothing, is driven by what Freud calls Eros and Thanatos. But really, Eros and Thanatos are one and the same. Eros, roughly, means the libidinal drive. All "love" drives derive from this. Thanatos, roughly, means the death drive, and all destructive drives derive from it. The drive of life is the chaotic expansion of life through love (Eros) and violence (Thanatos); and love and violence are two aspects of the same thing, which is the spread of life, being, and meaning until it occupies all of the nothing. Once it has occupied the nothing, it withdraws and dies and redefines itself. And then the expansion cycles again.
The Id as the Face of God
The self is only marginally aware of what is going on with this process. For the most part, it is shielded from the relentless beauty and brutality of the expansion and self-destruction that takes place with all life. To look into the Id is to look upon the face of God. The truth is that, while Freud viewed the Id as the most primal aspect of the human being (and yes, in some sense it is), it is also the highest forum of human experience. One experiences spirituality when one is in contact with the Id. When we mistake the projected rules of the Superego for spiritual guidance, what follows is misery. When we quiet the ego, Dasein absorbs our selves. The Id then to comes forth; spiritual understanding occurs.