Thus the connection of the Father in the Son, and of the Son in the Paraclete, produces three coherent Persons, who are yet distinct One from Another. These three are one [thing], not one [Person], as it is said, 'I and my Father are One,' in respect of unity of substance not singularity of number.

The Writings of Quintus Sept. Flor. Tertullianus (ed. 1884)