First of all, then, I shall propose to inquire by what criterion the soul can, according to its nature, be apprehended; then by what means it can be proved to exist; thereafter, whether it is a substance or an accident; then consequently on these points, whether it is a body or is incorporeal; then, whether it is simple or compound; next, whether it is mortal or immortal; and finally, whether it is rational or irrational.