natural deduction metalanguage, practical foundations
type theory (dependent, intensional, observational type theory, homotopy type theory)
computational trinitarianism = propositions as types +programs as proofs +relation type theory/category theory
Coherent logic is a fragment of (finitary) first-order logic which allows only the connectives and quantifiers
$\wedge$ (and),
$\vee$ (or),
$\top$ (true),
$\bot$ (false),
$\exists$ (existential quantifier).
A coherent formula is a formula in coherent logic.
A coherent sequent is a sequent of the form $\varphi \vdash \psi$, where $\varphi$ and $\psi$ are coherent formulas, possibly with free variables $x_1,\dots,x_n$.
In full first-order logic, such a sequent is equivalent to the single formula
(in the empty context). Of course, this latter formula is not coherent, but this shows that when we deal with coherent sequents rather than merely formulas, it can also be thought of as allowing one instance of $\Rightarrow$ and a string of $\forall$s at the very outer level of a formula.
Coherent logic (including sequents, as above) is the internal logic of a coherent category. The classifying topos of a coherent theory is a coherent topos.
Any (finitary) algebraic theory is coherent.
A good example of a coherent theory that is not algebraic (in any of the usual senses, although it comes from algebra) is the theory of a local ring; a similar example is the theory of a discrete field.
The theory of a total order is coherent, though also not algebraic. The theory of a partial order is essentially algebraic, but the totality axiom $\vdash_{x,y} (x\le y) \vee (y\le x)$ is coherent but not essentially algebraic.
The theory of a linear order is (seemingly) not coherent if we use the “connectedness” axiom $(x\nless y), (y\nless x) \vdash (x=y)$, which is not coherent since negation is not allowed in coherent formulas. We can express one outer negation, however, as in the irreflexivity axiom $(x\lt x)\vdash \bot$. Another solution is to use the “trichotomy” axiom $\top \vdash (x=y) \vee (x\lt y) \vee (y\lt x)$ instead, in order to get an axiomatisation of “coherent” linear orderings.
Coherent logic has many pleasing properties.
Every finitary first-order theory is equivalent, over classical logic, to a coherent theory. This theory is called its Morleyization and can be obtained by adding new relations representing each first-order formula and its negation, with axioms that guarantee (over classical logic) these relations are interpreted correctly (using the facts that $(P\Rightarrow Q) \dashv\vdash (\neg P \vee Q)$ and $(\forall x, P) \dashv\vdash (\neg \exists x, \neg P)$ in classical logic). See D1.5.9 in Sketches of an Elephant, or Prop. 3.2.8 in Makkai-Paré.
By (one of the theorems called) Deligne's theorem, every coherent topos has enough points. In particular, this applies to the classifying toposes of coherent theories. It follows that models in Set are sufficient to detect provability in coherent logic. By Morleyization, we can obtain from this the classical completeness theorem for first-order logic?. See for instance 6.2.2 in Makkai-Reyes.
Coherent logic also satisfies a definability theorem: if a relation can be constructed in every Set-model of a coherent theory $T$, in a natural way, then that relation is named by some coherent formula in $T$. See chapter 7 of Makkai-Reyes or D3.5.1 in Sketches of an Elephant.
It follows that if a morphism of coherent theories (i.e. an interpretation of one coherent theory in another) induces an equivalence of their categories of models in $Set$, then it is a Morita equivalence of theories (i.e. induces an equivalence of classifying toposes, hence an equivalence of categories of models in all Grothendieck toposes). This is called conceptual completeness; see 7.1.8 in Makkai-Reyes or D3.5.9 in Sketches of an Elephant. (Note, though, that two coherent theories can have equivalent categories of models in $Set$ without being Morita equivalent, if the former equivalence is not induced by a morphism of theories; see for instance D3.5.4 in the Elephant.)
However, here is a property which one might expect coherent theories to have, but which they do not.
Sometimes coherent logic is called geometric logic, but that term is now more commonly used for the analogous fragment of infinitary logic which allows disjunctions over arbitrary sets (though still only finitary conjunctions). See geometric logic.
Conversely, geometric logic is sometimes called coherent , e.g. in Reyes (1977), so that coherent logic in the nLab terminology corresponds to the finitary fragment only.
Occasionally the existential quantifiers in coherent logic are further restricted to range only over finitely presented types.
finitely complete category, cartesian functor, cartesian logic, cartesian theory
regular category, regular functor, regular logic, regular theory, regular coverage, regular topos
coherent category, coherent functor, coherent logic, coherent theory, coherent coverage, coherent topos
geometric category, geometric functor, geometric logic, geometric theory
A survey of results on geometric and coherent logic is in
Else consider the monographs
Michael Makkai and Gonzalo Reyes, First Order Categorical Logic: Model-theoretical methods in the theory of topoi and related categories, Springer-Verlag, 1977.
Michael Makkai and Robert Paré, Accessible categories
Peter Johnstone, Sketches of an Elephant vol 2. Part D
Jiri Adamek and Jiri Rosicky, Locally presentable and accessible categories