But if our union with the To Elizabeth, th June. . 'Finally, as I consider that it is very necessary to have understood, once in one's lifetime, the principles of metaphysics, since they are what provide us with knowledge of God and our soul, I think too, however, that it would be extremely harmful to occupy our mind often in meditating upon them, since it could not then attend so effectively to the work of imagination and the senses; but that the best course is merely to retain in memory and belief conclusions once arrived at, and thenceforth to employ the rest of the time one can devote to study to thoughts in which the understanding acts along with the imagination and the senses.'