Let be a given profunctor. We regard it as a category, fully containing both
and
, with additional heteromorphisms
.
Objects of will be regarded as abstract situations (we are given this and that perhaps with certain elementary conditions), objects of
will be called models, and a heteromorphism
is regarded as an interpretation of situation
in model
.
We define a formal logic by introducing the formulas as diagrams in
of shape of a directed rooted tree, which doesn’t have infinite path (but may have infinite branches at any node).
Definition. Such a given tree is said to be valid in a given model
(in notation
), if
Eve
has a winning strategy against Adam
in the following game:
– Adam
starts the game by saying an interpretation of the root
of
in the model. [
Eve
automatically wins if there is no such interpretation.]
– After a move , the other player has to find a next arrow
in the tree and say an interpretation
making the triangle commutative:
.
– If a player cannot make a next move (either a leaf is reached, or there is no such interpretation), loses.
For example, if a tree consists of a single arrow , then
means that
is an
-injective object (all interpretations of
in
extend to interpretations of
along
), in other words, that
has the left lifting property with respect to
, where
is an initial object of
.
In general, the moves of ‘), the branches which he can choose -on even levels-, correspond to conjunction (‘
‘), and dually the moves of
Eve
correspond to existential choices (‘‘), and her branches -on odd levels- to disjunction (‘
‘).
We define a first order situation for a fixed similarity type , to be a pair
where
is any set and
is a set of elementary formulas (using the given relations symbols or equality for terms) built up by variables from
.
We can simply set be the preorder category induced by
if (
and
).
An interpretation in a
type first order model
is a mapping
such that each element of
holds under the evaluation
.