Summa Theologica

Actuality refers to what a thing is. Potentiality refers to what it is possible for a thing to be. A thing may hold many possibilities within it, which an agent can bring into actuality by acting upon it. For example, a match has the potentiality to be lit. It becomes actually lit by someone striking it.

God alone is pure act, with no potentiality. He causes things, and is not caused by anything. He is unchangeable and lacks nothing. 

An inclination toward something, from the Latin appetere, to strive after, to aim for. An important term in Aquinas’ moral theology and theory of the passions of man. The appetites are what drive us to pursue certain goals and make moral choices. 

We can explain a thing in terms of the various causes that produce it. For Aquinas, cause has a broader sense than it does in modern language and refers to the underlying principles of things. Following Aristotle, Aquinas speaks of four causes:

Efficient cause: the primary source of change in a thing. The efficient cause of a house is the builder.

Material cause: That out of which a thing comes to be. The material cause of the house is bricks, mortar, glass, etc.

Formal cause: The determining shape or design of a thing. The formal cause of the house is the blueprint.

Final cause: That for the sake of which something is done. The final cause of the house is to provide shelter.

A term belonging to moral theology, from the Latin concupere (to desire ardently), it denotes the tendency toward sin, often associated with lust or sexual desire. However, Aquinas treats the term in a more neutral light, as a strong tendency arising in the sensitive appetite and which can influence the will.

There is natural concupiscence—for things necessary to life, like food and sex. These man shares with animals. Only man desires things beyond his natural needs, such as wealth, fame, etc. When they are out of proportion with reason, these desires are disordered and are called cupidity.

The purpose of goal for the sake of which something is done. The final end of human beings and of the whole universe is union with God. There are any number of subsidiary ends which human beings may pursue on the way to this final end. 

The definition of a thing; what a thing is. It is distinct from existence, which means that fact of a thing existing. In God, essence and existence are the same, because he is being itself.

The principle that determines what a thing is; in material things, it is joined to matter. Matter individuates the form it inheres in. For example, the form of dog is the basic design that makes dogs what they are. The matter is what makes a dog this individual dog, e.g., Fido. 

The mind or faculty of understanding, which exists in man, the angels, and God. For Aquinas, following Aristotle, there are two parts to the intellect: the active (or agent) intellect, and the passive (or possible) intellect. The passive intellect receives information from the senses. The active intellect then makes these sense impressions intelligible by abstracting the essence of the perceived thing. Thus, the act of understanding is both passive/receptive and active/dynamic. 

From Latin meaning literally “whatness.” Used in a similar sense to essence. The basic definition of a thing, what it shares with other things of similar type. For example, we might speak of the quiddity of dog, the qualities that all dogs share. 

For Plato and Aristotle, soul meant the principle that gives life to living things—that which animates them (from Latin anima). Aquinas follows this definition, calling the “act of the living body.” Basically, the soul is the non-material part of man. It includes the sense powers and the intellect. For Aquinas, a human being is a union of body and soul, where both elements are essential to the definition of man. Yet Aquinas seeks to reconcile Aristotle’s definition of the soul with the Christian (and Platonic) belief that the soul survives the death of the body.