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
.
