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 similar to ‘schema’ can be found in his work, namely sensorium commune.
What are the main results of this historical survey? Firstly, the analysis of uses of the notion of schema in the philosophical literature before Kant shows how it does not possess a unique meaning but is employed in different, albeit related, ways. Secondly, although this notion has manifold connotations (shape, figure, example, form) and applications (logical, rhetorical, ontological, biological), it can be generally regarded as a function of mediation and order. Thirdly, what is remarkable is that the expression ‘schema’ and its cognates in other languages is often part of more complex, linguistic constructions (such as schema perceptionis) or it is used more as a way to define terms rather than something that requires explicit definition itself. It is regarded as a synonym for a figure of speech, “life of pretence”, arguments, or it is used to describe the process of the constitution of ideas and then referred to as schema perceptionis (i.e. in reference to perceptions, and not as an autonomous term). ‘Schema’ is not treated as a separate topic of its own, but rather it is used in the definition and explanations of other concepts and ideas. This lack of interest in defining the notion has probably led to the multiplicity of its meanings in different contexts. As we will see in the subsequent chapters there are similarities between Kant’s conception and certain aspects of the views of predecessors such as Thomasius, Darjes and Tetens46.
Kant might be accused of having disregarded the accounts of his predecessors concerning the notion of schemata. In the Critique of Pure Reason he makes several references to Plato, Aristotle, Bacon and Tetens, but none of these are devoted to the topic of schemata. However, Kant will build a doctrine of schemata whose meaning finds, in the Critique of Pure Reason, its greater epistemic and philosophical expression and complexity. It can be used as a main element for explaining the possibility of objective knowledge and the relation between sensibility and understanding. I shall show that Kant has the merit to have provided schemata with a definition (or definitions) and a precise role in his thought.←40 | 41→
4 From skhein, aorist of ekhō (whose general meanings are: to have, to hold, to keep).
5 “[…] οὐ γὰρ τέχνῃ οὐδ’ ἐπιστήµῃ περὶ Ὁµήρου λέγεις ἃ λέγεις, ἀλλὰ θείᾳ µοίρᾳ καὶ κατοκωχῇ, ὥσπερ οἱ κορυβαντιῶντες ἐκείνου µόνου αἰσθάνονται τοῦ µέλους ὀξέως ὃ ἂν ᾖ τοῦ θεοῦ ἐξ ὅτου ἂν κατέχωνται, καὶ εἰς ἐκεῖνο τὸ µέλος καὶ σχηµάτων καὶ ῥηµάτων εὐποροῦσι, τῶν δὲ ἄλλων οὐ φροντιζουσιν.”
6 Catoni (Catoni 2013), focuses on the meaning of ‘schema’ in relation to the ancient view of arts and dances as processes of sensibilisation of what belongs not to sensibility.
7 “[…] ταῦτα γὰρ οὔτε ῥᾴδια φύεσθαι, γµενάότε, καὶ τροφῆς καὶ παιδείας τυχόντα ἧς δεῖ, τοὺς πλείστους αὐτῶνκαὶ χείρους