In Nestor and again in Proteus, Stephen thinks of the human soul as the "form of forms," capable of containing the intelligible essences of everything in the world. This thought is inspired by Aristotle's chief psychological work, De Anima (On the Soul), with some help from his Metaphysics.
In Nestor Stephen has been thinking of his "mind's darkness" as something hidden, "reluctant, shy of brightness," but suddenly, inspired perhaps by the "glowlamps" within the cavernous gloom of the St. Geneviève reading room, he shifts to metaphors of light: "Thought is the thought of thought. Tranquil brightness. The soul is in a manner all that is: the soul is the form of forms. Tranquility sudden, vast, candescent: form of forms."
In an important passage of On the Soul, Aristotle uses light as a metaphor for an aspect of Mind that is eternally active and perfectly capacious. He has been applying his theory of potentiality and actuality to the various forms of sensation and thought. Human thought, he says, must be understood as something that emerges from possible into active existence. The material from which thought is formed—the "potential," "possible," or "passive" component of thought—seems to derive in some way from sensory impressions. But what "actual" force, or "activity," or "agent," works upon the materials of sensation to produce ideas?
Aristotle addresses the question very briefly and metaphorically, saying that "Mind in the passive sense is such because it becomes all things, but mind has another aspect in that it makes all things; this is a kind of positive state like light; for in a sense light makes potential into actual colours. Mind in this sense is separable, impassive and unmixed, since it is essentially an activity; for the agent is always superior to the patient, and the originating cause to the matter. . . . Mind does not think intermittently. When isolated it is its true self and nothing more, and this alone is immortal and everlasting (we do not remember because, while mind in this sense cannot be acted upon, mind in the passive sense is perishable), and without this nothing thinks" (430a15-25, trans. W.S. Hett).
The active intellect, then, is "immortal," unlike other parts of the human mind. It is "separable" from matter, "unmixed" with anything else. Some commentators, Stephen among them, associate its unceasing and pure intellectual activity with a description of the divine mind that Aristotle ventures in the Metaphysics. The Prime Mover, he says there, is thought thinking itself, and Stephen echoes this definition: "Thought is the thought of thought." The divine or godlike force called the active intellect acts upon human brains to produce thought in them, so moments of penetrating insight can be thought of as "Tranquil brightness . . . sudden, vast, candescent."
Stephen's thought that "the soul is the form of forms" reflects Aristotle's analysis, in On the Soul, of how the human mind actualizes the forms of various intelligible objects within itself. The soul (psyche) is the "form" of the body. Sensation is one form that that form takes, by receiving and processing the sensible forms of objects in the external world. Objects also have intelligible forms and the rational soul can assimilate these forms as well, so that the soul (potentially, temporarily, in the act of thinking) "becomes all things." Stephen seems to be combining this epistemological analysis with the metaphysical language devoted to the active intellect to draw the conclusion that "The soul is in a manner all that is." Like the vast mind called the Prime Mover, in which all things are endlessly contemplated, the human mind can contain all of reality.
The relevance of this speculation, in the mind of the would-be artist, to an epic and encyclopedic literary work like Ulysses is obvious. In A Portrait of the Artist Stephen used a few phrases culled from Thomas Aquinas to articulate three qualities of the "esthetic image." In Ulysses he goes back to Aquinas' inspiration, Aristotle, to think about how the artistic mind, and by extension its literary offspring, can encompass everything.
The active intellect was the subject of intense speculation in medieval philosophy, among those who sought to combine Aristotelianism and monotheistic faith. Aquinas defined it as a power within the individual psyche, because he believed in an individual soul that was God-created, God-like, and immortal. But Aristotle did not believe in the survival of the individual mind. He says that the active intellect is "immortal and everlasting," but those very qualities distinguish it from everything organic. "When isolated it is its true self and nothing more": it exists apart from human beings, acts upon their minds to produce thought in them, and survives when they perish. It seems to be some kind of transcendent force, perpetually active, and not an "intermittently" active human faculty. Early Peripatetic and Neoplatonic commentators, some medieval Arabs and Jews like Avicenna, Averroes, and Moses Maimonides, and some medieval Christians as well, responded more perceptively to this aspect of Aristotle's psychology than Aquinas did. They identified the "active" Mind variously as God, or a divine emanation, or a universal mind or soul.