of experience with a new order. It acts on impressions comparing them to each other and analysing, breaking down each one into its elements in order to produce simpler representations, which are not evident at first sight of the complex given perceptions. In this process, past representations (phantasmata) can be recalled but they are not sufficient to determine objects of thought (ideas), regarded as unities related to each other through thinking and reasoning. However, the representations provided by imagination are in themselves only a sort of matter provided by sensations and they still lack form:
“Representations turn into ideas and thoughts, but considered in themselves, they are not. The image of the moon is only the material for the idea of the moon; it still lacks form: the idea contains, beyond the representation, a consciousness, perception and distinction and presupposes comparisons and judgements, when regarded as an idea of a certain object.” (Tetens 1777, I, p. 26, transl. L.S.)39
This leads us to the second main aspect of his account: a superior faculty, the understanding, is needed to unite the representations as a whole, providing them with an intelligible and objective character.←36 | 37→
The faculty of imagination, according to Tetens, places itself between sensibility and understanding, between passivity and activity, therefore sharing similarities with the view of Kant. However, their accounts are not fully identical because Kant asserts that the productive imagination is a synthetic a priori function. As de Vleeschauwer underlines: “[…] the reproductive function is […] examined in its constitution and psychological activity and it is distinct from the way in which Kant dealt with it, mostly for the absence of the reference to a synthetic capacity.” (de Vleeschauwer 1934–1937, II, p. 97, transl. L.S.)40
Moreover, through his doctrine of the constitution of knowledge, Tetens opposes empiricism, relying on non-sensory functions (understanding, the soul, and apperception) as necessary conditions for developing an objective knowledge and unifying ideas: sensible data provided by experience need a common referent, an understanding, in order to be united and compared. Since this obscure unity is not in itself an impression, but a core unity to which all the impressions are referred, it has to be thought of as something immaterial, which holds an obscure feeling of awareness of its permanence, of its identity:
“[…] the idea or representation of my ´I` is not a collection of individual representations that our imagination might have turned into a whole just like it unifies the individual representations of soldiers into a representation of one regiment. That unification lies in the impression itself, in nature, and not in a combination that makes itself. For this reason a representation of ne subject with different features arises, that is, a representation that immediately arises from the impression, must be thought in this way and turned into an idea such that the common human understanding actually does from it in this way.” (Tetens 1777, pp. 394–5, transl. Watkins 2009, pp. 370–1)41
Through his doctrine of the ‘I’, Tetens distances himself from empiricism. As Thiel declares, three notions of the self can be distinguished in Tetens’s account. Firstly, the empirical, psychological self of the inner sense; secondly, the metaphysical ←37 | 38→self, as an immaterial entity that intends to collect all powers and operations of the human being; finally, the unitary self as a condition of the mental activity. In this third connotation the ‘I’ is regarded as a non-empirical function, as a necessary condition to develop an objective knowledge and unify ideas. Since each experience is related to this obscure but always present unity, Tetens rejected the Humean doctrine of the ‘I’ as a stream of sensations. The ‘I’ cannot be considered as a mere sum of representations, because the condition required to have them lies exactly in their reference to a common unity distinguished from them:
“[…] in order to have representations of external objects, an activity of judging or forming propositions is required, and that in order to be able to do the latter, an activity of distinguishing between the external thing, the representation, and one’s own self is required. […] it is the notion of something that we have to think in order to be able to explain representations of external objects. Without such a notion of the self, as distinct from the representations of other things, the possibility of forming propositions about the existence of external things could not even be entertained. This notion is a requirement of thought.” (Thiel 2018, pp. 70–71)
Tetens connects his epistemological dualism with a traditional Cartesian, ontological dualism. In his view, human nature consists of both a material and an immaterial aspect which form a union or interact with one another: “[…] in the human essence, beyond the bodily organ, a simple, non-corporeal essence, an actual, substantial unity can be found, which is properly the thing that perceives, thinks and wants.” (Tetens 1777, II, p. 210 transl. L.S.)42
More specifically, he explains this dualistic interactionism by means of two basic propositions:
“[…] with each manifestation of the soul a certain inner part of our body acts; we can call this part the brain, sensorium commune, the organ of the soul, schema perceptionis or what else. The other basic truth is: there is beyond our bodily organs of the soul an essence, that acts in conjunction with each thing, but that is in itself an autonomous, permanent substance, that we call soul in a psychological meaning or our ‘I’.” (Tetens 1777, II p. 158, transl. L.S.)43←38 | 39→
While each mental process is causally connected to a bodily process (as the first proposition states), the unity of representations also requires the action of an immaterial self or ‘I’ (as per the second proposition)44. That is not a claim provided with necessity, but a reasonable hypothesis that cannot be falsified or demonstrated through observation. Tetens describes the first sentence as clearly empirical and claims that the second one requires “more reasoning”- “mehr Raisonnement” (Tetens 1777, II, p. 158, transl. L.S.) for becoming certain but emphasises at the same time that both propositions use concepts derived from sensation (ibid.) The evidence for the first sentence is, according to Tetens, clear and almost unnecessary to state: “Physiology and psychology have collected so many facts, which provide evidence of the modifications of the brain in contemporaneity with all modifications of the soul, that this proof can be considered without any doubt.” (Tetens 1777, II, pp. 159, transl. L.S.)45
Against this background Tetens regards the schema perceptionis not only as something that mediates between sensibility and understanding, but also as the physical centre of unification of all the data of experience. Unfortunately, observation cannot inquire fully and completely the nature of these ideas. It is only possible to affirm that experience teaches that our organs are constituted by nerves, in which it is probable (but not observable), that a fluid matter or vital spirits flow. In his view, this process provides the physiological correlate or basis for ideas, which are therefore called “material ideas” - “materielle Ideen” - (Tetens 1777, I, p. vii, transl. L.S.). The existence of such ideas is postulated as reasonable hypothesis. What is important, however, is that material ideas, just like representations as they are observed through inner sense, need to be unified. That is precisely what the sensorium commune or the schema perceptionis does. The interpretation of schema as mediating function linking passivity and activity, senses and cognition opens out one path towards Kant’s epistemic use of the notion. But Tetens was not the author of this notion of schema: Darjes, as it has already been stressed, uses this noun in an epistemic sense also. Bonnet although he himself does not use the word ‘schema’, plays a relevant role as well in the ←39 | 40→development of the research on a middle function between understanding and sensibility and an equivalent or at least concept
