Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
Do not think that what we have laid down in the preceding chapters on the importance, obscurity, and difficulty of the subject, and its unsuitableness for communication to ordinary persons, includes the doctrine of God's incorporeality and His exemption from all affections (πάθη). This is not the case. For in the same way as all people must be informed, and even children must be trained in the belief that God is One, and that none besides Him is to be worshipped, so must all be taught by simple authority that God is incorporeal; that there is no similarity in any way whatsoever between Him and His creatures: that His existence is not like the existence of His creatures, His life not like that of any living being, His wisdom not like the wisdom of the wisest of men; and that the difference between Him and His creatures is not merely quantitative, but absolute [as between two individuals of two different classes]: I mean to say that all must understand that our wisdom and His, or our power and His do not differ quantitatively or qualitatively, or in a similar manner; for two things, of which the one is strong and the other weak, are necessarily similar, belong to the same class, and can be included in one definition. The same is the case with an other comparisons: they can only be made between two things belonging to the same class, as has been shown in works on Natural Science. Anything predicated of God is totally different from our attributes; no definition can comprehend both; therefore His existence and that of any other being totally differ from each other, and the term existence is applied to both homonymously, as I shall explain.
This suffices for the guidance of children and of ordinary persons who must believe that there is a Being existing, perfect, incorporeal, not inherent in a body as a force in it-God, who is above all kinds of deficiency, above A affections. But the question concerning the attributes of God, their inadmissibility, and the meaning of those attributes which are ascribed to Him; concerning the Creation, His Providence, in providing for everything; concerning His will, His perception, His knowledge of everything; concerning prophecy and its various degrees: concerning the meaning of His names which imply the idea of unity, though they are more than one; all these things are very difficult problems, the true "Secrets of the Law" the
[paragraph continues] "secrets" mentioned so frequently in the books of the Prophets, and in the words of our Teachers, the subjects of which we should only mention the headings of the chapters, as we have already stated, and only in the presence of a person satisfying the above-named conditions.
That God is incorporeal, that He cannot be compared with His creatures, that He is not subject to external influence; these are things which must be explained to every one according to his capacity, and they must be taught by way of tradition to children and women, to the stupid and ignorant, as they are taught that God is One, that He is eternal, and that He alone is to be worshipped. Without incorporeality there is no unity, for a corporeal thing is in the first case not simple, but composed of matter and form which are two separate things by definition, and secondly, as it has extension it is also divisible. When persons have received this doctrine, and have been trained in this belief, and are in consequence at a loss to reconcile it with the writings of the Prophets, the meaning of the latter must be made clear and explained to them by pointing out the homonymity and the figurative application of certain terms discussed in this part of the work. Their belief in the unity of God and in the words of the Prophets will then be a true and perfect belief.
Those who are not sufficiently intelligent to comprehend the true interpretation of these passages in the Bible, or to understand that the same term admits of two different interpretations, may simply be told that the scriptural passage is clearly understood by the wise, but that they should content themselves with knowing that God is incorporeal, that He is never subject to external influence, as passivity implies a change, while God is entirely free from all change, that He cannot be compared to anything besides Himself, that no definition includes Him together with any other being, that the words of the Prophets are true, and that difficulties met with may be explained on this principle. This may suffice for that class of persons, and it is not proper to leave them in the belief that God is corporeal, or that He has any of the properties of material objects, just as there is no need to leave them in the belief that God does not exist, that there are more Gods than one, or that any other being may be worshipped.