Authenticity of this letter has been disputed (see Wikipedia).
Martin Buber praised this letter in The Eclipse of God for its poetic approach to philosophy. This paragraph from the letter expresses the view that true philosophical wisdom should be dynamically held in the mind rather than written down!
The same applies to straight as well as to circular form, to colours, to the good, the, beautiful, the just, to all bodies whether manufactured or coming into being in the course of nature, to fire, water, and all such things, to every living being, to character in souls, and to all things done and suffered. For in the case of all these, no one, if he has not some how or other got hold of the four things first mentioned, can ever be completely a partaker of knowledge of the fifth. Further, on account of the weakness of language, these (i.e., the four) attempt to show what each thing is like, not less than what each thing is. For this reason no man of intelligence will venture to express his philosophical views in language, especially not in language that is unchangeable, which is true of that which is set down in written characters.