Simple

SIMPLE. - It is that which has no parts and is therefore not divisible.

Since indivisibility is unity ("nihil enim est aliud unum quam ens indivisum": St. Thomas, 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 positive perfection or quality; but since human knowledge begins from the experience of the corporeal world, and the body is composite, the concept of the simple is had by negation of partibility or composition: it is therefore relative to that of the parts denied. Absolutely simple 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, involves instead a composition of act and potency (according to Thomism, of existence and essence). Spiritual reality is, however, relatively simple inasmuch as it has no quantitative parts; and this simplicity proceeds, always in the Thomistic conception, from its not having a composition of essential parts (matter and form): that is, from its being pure form. The body, composed of matter and form, is extended and as such is also composed of quantitative parts.

Sofia Vanni-Rovighi