H. P. Blavatsky
From Lucifer 1.2 (October 1887): 96; Collected Writings 8:109
Will is the exclusive possession of man on this our plane of consciousness. It divides him from the brute in whom instinctive desire only is active.
Desire, in its widest application, is the one creative force in the Universe. In this sense it is indistinguishable from Will; but we men never know desire under this form while we remain only men. Therefore Will and Desire are here considered as opposed.
Thus Will is the offspring of the Divine, the God in man; Desire the motive power of the animal life.
Most of men live in and by desire, mistaking it for will. But he who would achieve must separate will from desire, and make his will the ruler; for desire is unstable and ever changing, while will is steady and constant.
Both will and desire are absolute creators, forming the man himself and his surroundings. But will creates intelligently—desire blindly and unconsciously. The man, therefore, makes himself in the image of his desires, unless he creates himself in the likeness of the Divine, through his will, the child of the light.
His task is twofold: to awaken the will, to strengthen it by use and conquest, to make it absolute ruler within his body; and, parallel with this, to purify desire.
Knowledge and will are the tools for the accomplishment of this purification.