In a few very dense pages, Alain de Libera summarizes a number of key theses extracted from the works of Thomas Aquinas pertaining to the act of thought (see also “The Subject” In Medieval Times; Origins of a Subject-Agent). According to de Libera, these principles — which represented a significant departure from Augustine’s insistence that the human soul should not be viewed as a “subject” in the sense of something standing under something else — attained a wide currency in Latin scholasticism. They laid the groundwork for the modern notion of “the subject” as active mind and ego.
“Thought is an action (actio) or an operation (operatio) called ‘intellectual’ (intellectualis) or ‘intelligible’ (intelligibilis) because it is the deed of intellect and treats of the intelligible, and unites these two dimensions in its proper actuality. Intellectual has two senses: subjective and causal. Contemplation, also called theoretical thought, the knowledge of the intelligible, is intellectual because it takes place in the intellect itself, which is to say that, relative to the body, it is atopical or utopical [without place], because the intellect itself is not located in the body; the other actions called intellectual are so in a causal sense; they are called such because they are directed or imposed, that is to say commanded, by the intellect and executed by means of a bodily instrument — with respect to which, in distinction from the act of thought — they are localizable and localized: this is the case, for example, with walking and riding, two actions called imposed.”
“…There are two kinds of actions: one remains internal to the agent, begins and ends in it (it is called: manens [remaining] or consistens [consisting] or quiescens in agente [resting in the agent]); the other is exercised on another thing or an exterior matter (it is called: exiens [coming out] or progrediens [moving forward] or tendens [tending] or transiens in alterum [passing into another] or in materiam exteriorem [into the matter of exteriors]). This duality prolongs the Aristotelian distinction between immanent action and transitive action…. The distinction, massively utilized to theorize the difference between the psychic (where immanent causality reigns) and the physical (where the transitive reigns), is also applied within the physical sphere, notably to light….”
“…Only that which is in act acts (Nihil agit nisi secundum quod est actu). This fundamental thesis, which lays the foundation of the articulation between actio and actus [action and act], introduces itself in diverse other formulations, such as: Omne agens agit, inquantum actu est [every agent acts, insofar as it actually is], or Unumquodque agit secundum quod est actu [each one acts according to what it actually is]. We will call it ‘the principle of the actuality of the agent’.”
“Numerous principles arise from this or assume its validity. This is the case with [the principle that] that by which something first operates is the form of the operator; the principle of the subjection of action in the power of the agent… and the subjective principle of action [actions belong to something standing under them]” (Archéologie du sujet vol. 3 part 1, pp. 53-56, my translation). De Libera goes on to mention additional principles such as “attribution of action to the principal agent” (pp. 56-57); “action is a function of the being of the agent” (p. 57); “determination of action by the nature of the agent” (ibid); “determination of action by act” (ibid); and “actuality is a determination of the act of an agent” (ibid).
The bottom line of all of this seems to be that thought is the action of an agent. Neither Aristotle nor Augustine treated thought in this way or had this kind of view of action and agency, but a long medieval and modern heritage makes it seem like common sense to many people. Aristotle spoke of intellect as coming to us “from outside”. He was certainly very interested in practical doings, in process, and in being-at-work, but did not reduce these to the discrete “actions” of discrete “agents”. Activity, I want to say, is something different and broader than this. (See also Not Power and Action; Aristotelian Actualization; Aristotelian Subjectivity Revisited).