So do thou apprehend this Wisdom, o my Son, laying it to thine Heart, as a Mistress, and hiding it in the Treasury of thy Mind as a Jewel of Enlightenment. Consider a Dream, how it is unreal in Respect of thine Experience of the Objects of thy Waking Sense, but real also, both as it did in Fact impress thy Mind, and as it did express some Hunger of thy Secret Nature, as I have already shewed in this Letter. Consider the Play of the Chess, how its Law hath made for itself a Language and a Literature, yet it is but an arbitrary invention; without impinging (save as it operateth though Pleasure and Interest upon Minds) on any other Sphere soever of the Universe. Equally, Things called (vulgarly) Real and Material exist in the Universe of our Consciousness only by the Apprehension of their Images in mind through Sense; as, how is Colour Real or Material to a blind Man; or a Law mathematical true to him that is imbecile or demented? All Things therefore, even if unreal and irrational, nay, inconceivable and impossible (such as Iota in the Theorem of De Moivre), exist in one form or another; but the Reality of any, though in itself absolute, is in regard of its Relation with any other thing dependent upon the Intercourse and Language between them, conscious or unconscious. Consider Azote, that hath night Four Parts in Five of the Air, how it is not real to the Perception of any human Sense, but yet most real to our Lungs, diluting the Oxygen, by whose Love we were else violently combust. This is the Measure of Reality.