Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
SIMILARITY is based on a certain relation between two things: if between two things no relation can be found, there can be no similarity between them, and there is no relation between two things that have no similarity to each other; e.g., we do not say this heat is similar to that colour, or this voice is similar to that sweetness. This is self-evident. Since the existence of a relation between God and man, or between Him and other beings has been denied, similarity must likewise be denied. You must know that two things of the same kind--i.e., whose essential properties are the same, and which are distinguished from each other by greatness and smallness, strength and weakness, etc.--are necessarily similar, though different in this one way; e.g., a grain of mustard and the sphere of the fixed stars are similar as regards the three dimensions, although the one is exceedingly great, the other exceedingly small, the property of having [three] dimensions is the same in both: or the heat of wax melted by the sun and the heat of the element of fire, are similar as regards heat: although the heat is exceedingly great in the one case, and exceedingly small in the other, the existence of that quality (heat) is the same in both. Thus those who believe in the presence of essential attributes in God, viz., Existence, Life, Power, Wisdom, and Will, should know that these attributes, when applied to God, have not the same meaning as when applied to us, and that the difference does not only consist in magnitude, or in the degree of perfection, stability, and durability. It cannot be said, as they practically believe, that His existence is only more stable, His life more permanent, His power greater, His wisdom more perfect, and His will more general than ours, and that the same definition applies to both. This is in no way admissible, for the expression "more than" is used in comparing two things as regards a certain attribute predicated of both of them in exactly the same sense, and consequently implies similarity [between God and His creatures]. When they ascribe to God essential attributes, these so-called essential attributes should not have any similarity to the attributes of other things, and should according to their own opinion not be included in one of the same definition, just as there is no similarity between the essence of God and that of other beings. They do not follow this principle, for they hold that one definition may include them, and that, nevertheless, there is no similarity between them. Those who are familiar with the meaning of similarity will certainly understand that the term existence, when applied to God and to other beings, is perfectly homonymous. In like manner, the terms Wisdom, Power, Will, and Life are applied to God and to other beings by way of perfect homonymity, admitting
of no comparison whatever. Nor must you think that these attributes are employed as hybrid terms; for hybrid terms are such as are applied to two things which have a similarity to each other in respect to a certain property which is in both of them an accident, not an essential, constituent element. The attributes of God, however, are not considered as accidental by any intelligent person, while all attributes applied to man are accidents, according to the Mutakallemim. I am therefore at a loss to see how they can find any similarity [between the attributes of God and those of man]; how their definitions can be identical, and their significations the same! This is a decisive proof that there is, in no way or sense, anything common to the attributes predicated of God, and those used in reference to ourselves: they have only the same names, and nothing else is common to them. Such being the case, it is not proper to believe, on account of the use of the same attributes, that there is in God something additional to His essence, in the same way as attributes are joined to our essence. This is most important for those who understand it. Keep it in memory, and study it thoroughly in order to be well prepared for that which I am going to explain to you.