An inverse system is a collection of sets which are connected by mappings. We start off with the definitions before relating these to abstract properties of synthesis.

Definition: A directed set is a set T together with an ordering relation ≤ such that

(1) ≤ is a partial order, i.e. transitive, reflexive, anti-symmetric

(2) ≤ is directed, i.e. for any s, t ∈ T there is r ∈ T with s, t ≤ r

Definition: An inverse system indexed by T is a set D = {D_{s}|s ∈ T} together with a family of mappings F = {h_{st}|s ≥ t, h_{st} : D_{s} → D_{t}}. The mappings in F must satisfy the coherence requirement that if s ≥ t ≥ r, h_{tr} ◦ h_{st} = h_{sr}.

Interpretation of the index set: The index set represents some abstract properties of synthesis. The ‘synthesis of apprehension in intuition’ proceeds by a ’running through and holding together of the manifold’ and is thus a process that takes place in time. We may now think of an index s ∈ T as an interval of time available for the process of ’running through and holding together’. More formally, s can be taken to be a set of instants or events, ordered by a ‘precedes’ relation; the relation t ≤ s then stands for: t is a substructure of s. It is immediate that on this interpretation ≤ is a partial order. The directedness is related to what Kant called ‘the formal unity of the consciousness in the synthesis of the manifold of representations’ or ‘the necessary unity of self-consciousness, thus also of the synthesis of the manifold, through a common function of the mind for combining it in one representation’ – the requirement that ‘for any s, t ∈ T there is r ∈ T with s, t ≤ r’ creates the formal conditions for combining the syntheses executed during s and t in one representation, coded by r.

Interpretation of the D_{s} and the mappings h_{st} : D_{s} → D_{t}. An object in D_{s} can thought of as a possible ‘indeterminate object of empirical intuition’ synthesised in the interval s. If s ≥ t, the mapping h_{st} : D_{s} → D_{t} expresses a consistency requirement: if d ∈ D_{s} represents an indeterminate object of empirical intuition synthesised in interval s, so that a particular manifold of features can be ‘run through and held together’ during s, some indeterminate object of empirical intuition must already be synthesisable by ‘running through and holding together’ in interval t, e.g. by combining a subset of the features characaterising d. This interpretation justifies the coherence condition s ≥ t ≥ r, h_{tr} ◦ h_{st} = h_{sr}: the synthesis obtained from first restricting the interval available for ‘running through and holding together’ to interval t, and then to interval r should not differ from the synthesis obtained by restricting to r directly.

We do not put any further requirements on the mappings h_{st} : D_{s} → D_{t}, such as surjectivity or injectivity. Some indeterminate object of experience in D_{t} may have disappeared in D_{s}: more time for ‘running through and holding together’ may actually yield fewer features that can be combined. Thus we do not require the mappings to be surjective. It may also happen that an indeterminate object of experience in D_{t} corresponds to two or more of such objects in D_{s}, as when a building viewed from afar upon closer inspection turns out to be composed of two spatially separated buildings; thus the mappings need not be injective.

The interaction of the directedness of the index set and the mappings h_{st} is of some interest. If r ≥ s, t there are mappings h_{rs} : D_{r} → D_{s} and h_{rt} : D_{s} → D_{t}. Each ‘indeterminate object of empirical intuition’ in d ∈ D_{r} can be seen as a synthesis of such objects h_{rs}(d) ∈ D_{s} and h_{rt}(d) ∈ D_{t}. For example, the ‘manifold of a house’ can be viewed as synthesised from a ‘manifold of the front’ and a ‘manifold of the back’. The operation just described has some of the characteristics of the synthesis of reproduction in imagination: the fact that the front of the house can be unified with the back to produce a coherent object presupposes that the front can be reproduced as it is while we are staring at the back. The mappings h_{rs} : D_{r} → D_{s} and h_{rt} : D_{s} → D_{t} capture the idea that d ∈ D_{r} arises from reproductions of h_{rs}(d) and h_{rt}(d) in r.