SIMPLE. — That which has no parts and is therefore not divisible.
Since indivisibility is unity (« nihil enim est aliud unum quam ens indivisum »: s. Tommaso, De verit., q. 1, a. 1, c.) and unity is convertible with being (« Unum... convertitur cum ente »: id., De potentia, q. 9, a. 7), simplicity or indivisibility is a perfection or positive quality; but since human knowledge begins with experience of the corporeal world, and the body is composite, the concept of s. is obtained by negating divisibility or composition: it is therefore relative to that of the parts negated. Absolutely s. is that which has no parts at all, that which is wholly and purely being, pure act: God (v.). Every finite being, even if spiritual, on the other hand entails a composition of act and potency (according to Thomism, of existence and essence). Spiritual reality is nevertheless relatively s. insofar as it has no quantitative parts; and this simplicity proceeds, always according to the Thomistic conception, from not having a composition of essential parts (matter and form): that is, from being pure form. The body, composed of matter and form, is extended and as such also composed of quantitative parts.