# Contents

## Idea

In dependent type theory the type $\mathrm{sProp}$ is a type of strict propositions.

The type of strict proposition is used to formalize logic over type theory in dependent type theory without having to postulate a separate proposition judgment, since strict propositions are judgmentally proof irrelevant, and thus behave as propositions in traditional propositional logic.

## Definition

### The type of all strict propositions

The type of all strict propositions $\mathrm{sProp}$ in a dependent type theory could be presented either as a Russell universe or a Tarski universe. The difference between the two is that in the former, every strict proposition in the type theory is literally an element of the type of all strict propositions, while in the latter, elements of $\mathrm{sProp}$ are only indices of a type family $\Box$ of strict propositions; every strict proposition in the type theory is only essentially $\mathrm{sProp}$-small for weak Tarski universes or judgmentally equal to an $\Box P$ for $P:\mathrm{sProp}$ for strict Tarski universes.

#### As a Russell universe

As a Russell universe, the type of all strict propositions is given by the following rules:

Formation rules:

$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathrm{sProp} \; \mathrm{type}}$

Type reflection:

$\frac{\Gamma \vdash A:\mathrm{sProp}}{\Gamma \vdash A \; \mathrm{type}}$

Strictness for each type reflection

$\frac{\Gamma \vdash A:\mathrm{sProp}}{\Gamma, x:A, y:A \vdash x \equiv y:A}$

Introduction rule:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A, y:A \vdash x \equiv y:A}{\Gamma \vdash A:\mathrm{sProp}}$

Univalence:

$\frac{\Gamma \vdash A:\mathrm{sProp} \quad \Gamma \vdash B:\mathrm{sProp}} {\Gamma \vdash \mathrm{univalence}(A, B):\mathrm{Id}_\mathrm{sProp}(A, B) \simeq (A \simeq B)}$

#### As a Tarski universe

As a Tarski universe, the type of all propositions is given by the following rules:

Formation rules:

$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathrm{sProp} \; \mathrm{type}}$

Type reflection:

$\frac{\Gamma \vdash A:\mathrm{sProp}}{\Gamma \vdash \Box A \; \mathrm{type}}$

Strictness for each type reflection

$\frac{\Gamma \vdash A:\mathrm{sProp}}{\Gamma, x:A, y:A \vdash x \equiv y:A}$

Introduction rule:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A, y:A \vdash x \equiv y:A}{\Gamma \vdash A_\Omega:\mathrm{sProp}}$

Essential smallness of propositions (for weak types of all propositions) or judgmental equality (for strict types of all propositions):

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A, y:A \vdash x \equiv y:A}{\Gamma \vdash \delta_A:\Box A_\Omega \simeq A}\mathrm{weak} \quad \frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A, y:A \vdash x \equiv y:A}{\Gamma \vdash \Box A_\Omega \equiv A \; \mathrm{type}}\mathrm{strict}$

Univalence:

$\frac{\Gamma \vdash A:\mathrm{sProp} \quad \Gamma \vdash B:\mathrm{sProp}} {\Gamma \vdash \mathrm{univalence}(A, B):\mathrm{isEquiv}(\mathrm{transport}^\box(A, B))}$

#### The empty proposition and falsehood

To ensure that the type of all strict propositions isn’t a contractible type, one could include the following axiom, which states that there exists a strict proposition satisfying ex falso quodlibet:

$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathrm{axempty}:\sum_{\bot:\mathrm{sProp}} \prod_{C:\bot \to \mathrm{sProp}} \prod_{x:\bot} C(x)}\mathrm{Russell} \qquad \frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathrm{axempty}:\sum_{\bot:\mathrm{sProp}} \prod_{C:\Box \bot \to \mathrm{sProp}} \prod_{x:\Box \bot} \Box C(x)}\mathrm{Tarski}$