A set of Boolean functions is functionally complete if function composition using those operations can express every positive-arity Boolean function. The NAND gate alone suffices: tie its inputs to obtain negation, negate its output to obtain logical conjunction, and apply De Morgan's laws to obtain logical disjunction; then use disjunctive normal form.
Among sixteen binary Boolean truth functions, eight become functionally complete together with negation. The eight affine functions cannot express conjunction because composition preserves affinity. Each of the other eight truth tables has exactly one or three true rows. Negating the output if needed and selected inputs transforms its unique true row to the conjunction table. Thus conjunction and negation become definable and disjunctive normal form expresses every Boolean function.
Suppose a first-order language has logical implication as its only connective and has either or both of the quantifiers, but no primitive falsity. In a singleton structure with every atomic predicate true, all atomic equalities are also true, and structural induction makes every formula true. No formula of this language can therefore express a negated equality. Quantifiers cannot repair this failure of expressive completeness, even though they add syntactic operations beyond propositional connectives.
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Functional completeness is a concept in the field of mathematics and computer science, particularly in the study of logic and formal systems. It refers to a set of functions or operations that can be combined to express all possible functions within a given context or structure. In the context of logic, a set of logical connectives (like AND, OR, NOT) is said to be functionally complete if any possible logical expression can be formed using only those connectives.