Language acquisition is the process by which humans learn to perceive, understand, and produce language, requiring mastery of tools like phonology, syntax, and vocabulary, and relying on a syntactic principle called recursion to generate an infinite number of sentences. The process follows two key principles: speech perception always precedes speech production, and learning builds gradually, starting with distinguishing individual phonemes. It typically refers to first-language acquisition in infants (including simultaneous bilingual acquisition), but is distinct from second-language acquisition, which involves learning additional languages later in life. Historical theories on this "quintessential human trait" have ranged from Plato's belief in innate word-meaning mapping to empiricists like John Locke, who argued language emerges from sensory experience (nurture). In 1957, behaviorist B. F. Skinner proposed in Verbal Behavior that language is learned through operant conditioning, where successful use of a word is reinforced by rewards. While these early ideas laid the groundwork, modern linguists continue to debate how learners convert surface forms into abstract linguistic rules, making it a central question in cognitive science.