III.D8: Tendency Defined

Propensio est lætitia concomitante idea alicujus rei quæ per accidens causa est lætitiæ. 

Tendency is a joy accompanied by the idea of whatever thing which is a joy by accidental cause.

Spinoza is holding consistent that a tendency is not itself a primitive, but is rooted in the primitive of joy. This is important. Many might attribute a person's lack of power to avoid self-destructive behaviors to some tendency that seems structurally embedded. Instead, Spinoza argues that it is derived from joy but assigned an accidental cause in order to minimize identity with the tendency.