upon, as a governing source is above the material it works on. Knowledge [], in its being-at-work, is the same as the thing it knows, and while knowledge in potency comes first in time in any one knower, in the whole of things it does not take precedence even in time. This does not mean that at one time it thinks but at another time it does not think, but when separated it is just exactly what it is, and this alone is deathless and everlasting (though we have no memory, because this sort of intellect is not acted upon, while the sort that is acted upon is destructible), and without this nothing thinks. This has been referred to as one of "the most intensely studied sentences in the history of philosophy". In the Metaphysics, Aristotle wrote at more length on a similar subject and is often understood to have equated the active intellect with being the "unmoved mover" and God. Nevertheless, as Davidson remarks: Just what Aristotle meant by potential intellect and active intellect – terms not even explicit in the De anima and at best implied – and just how he understood the interaction between them remains moot to this day. Students of the history of philosophy continue to debate Aristotle's intent, particularly the question whether he considered the active intellect to be an aspect of the human soul or an entity existing independently of man. Post-Aristotelian usage New meanings of or energy Already in Aristotle's own works, the concept of a distinction between and was used in many ways, for example to describe the way striking metaphors work, or human happiness. Polybius about 150 BC, in his work the Histories uses Aristotle's word energeia in both an Aristotelian way and also to describe the "clarity and vividness" of things. Diodorus Siculus in 60-30 BC used the term in a very similar way to Polybius. However, Diodorus uses the term to denote qualities unique to individuals. Using the term in ways that could translated as 'vigor' or 'energy' (in a more modern sense); for society, 'practice' or 'custom'; for a thing, 'operation' or 'working'; like vigor in action. Platonism and neoplatonism Already in Plato it is found implicitly the notion of potency and act in his cosmological presentation of becoming () and forces (), linked to the ordering intellect, mainly in the description of the Demiurge and the