Arche is a Greek word with primary senses "beginning", "origin" or "source of action". (: from the beginning, οr : the original argument), and later first principle or element, first so used by Anaximander (Simplicius in Ph. 150.23), principles of knowledge (Aristot. Metaph. 995b8). By extension, it may mean "first place, power", "method of government", "empire, realm", "authorities" (in plural: ), "command". The first principle or element corresponds to the "ultimate underlying substance" and "ultimate undemonstrable principle". In the philosophical language of the archaic period (8th-6th century BC), arche (or archai) designates the source, origin or root of things that exist. In ancient Greek philosophy, Aristotle foregrounded the meaning of arche as the element or principle of a thing, which although undemonstrable and intangible in itself, provides the conditions of the possibility of that thing.