Traditional definitions of parts of speech based on meaning are criticized by linguists. Notional definitions assume a direct link between grammatical form and semantic meaning, but the relationship is complex. For example, "rejection" and "rejected" have similar meanings but different grammatical functions. Satisfactory definitions must identify grammatical properties that distinguish parts of speech. Words are classified according to their properties in phrases and clauses, not their meanings. Parts of speech fall into closed classes that are finite and stable, or open classes that constantly change.