Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
WHEN reading my present treatise, bear in mind that by "faith" we do not understand merely that which is uttered with the lips, but also that which is apprehended by the soul, the conviction that the object [of belief] is exactly as it is apprehended. If, as regards real or supposed truths, you content yourself with giving utterance to them in words, without apprehending them or believing in them, especially if you do not seek real truth, you have a very easy task as, in fact, you will find many ignorant people professing articles of faith without connecting any idea with them.
If, however, you have a desire to rise to a higher state, viz., that of reflection, and truly to hold the conviction that God is One and possesses true unity, without admitting plurality or divisibility in any sense whatever, you must understand that God has no essential attribute in any form or in any sense whatever, and that the rejection of corporeality implies the rejection of essential attributes. Those who believe that God is One, and that He has many attributes, declare the unity with their lips, and assume plurality in their thoughts. This is like the doctrine of the Christians, who say that He is one and He is three, and that the three are one. Of the same character is the doctrine of those who say that God is One, but that He has many attributes; and that He with His attributes is One, although they deny corporeality and affirm His most absolute freedom from matter; as if our
object were to seek forms of expression, not subjects of belief. For belief is only possible after the apprehension of a thing; it consists in the conviction that the thing apprehended has its existence beyond the mind [in reality] exactly as it is conceived in the mind. If in addition to this we are convinced that the thing cannot be different in any way from what we believe it to be, and that no reasonable argument can be found for the rejection of the belief or for the admission of any deviation from it, then the belief is true. Renounce desires and habits, follow your reason, and study what I am going to say in the chapters which follow on the rejection of the attributes; you will then be fully convinced of what we have said: you will be of those who truly conceive the Unity of God, not of those who utter it with their lips without thought, like men of whom it has been said, "Thou art near in their mouth, and far from their reins" (Jer. xii. 2). It is right that a man should belong to that class of men who have a conception of truth and understand it, though they do not speak of it. Thus the pious are advised and addressed, "Commune with your own heart upon your bed and be still. Selah." (Ps. iv. 5.)