At the heart of the Guide of the Perplexed is Maimonides’ conception of God. When we say that "God is one" every day [in the Sh’ma prayer], what do we mean by that statement? For many Jewish philosophers, Maimonides chief among them, this is the central question of Jewish philosophy. He argues that God is a perfect unity, not admitting of any plurality. God does not have parts, either literally or figuratively–no arms or legs, no back or front, no end or beginning. (One of the alternate names for God in Jewish discourse is Ein Sof [Without End].)
That also means that, in Aristotelian terms, one cannot actually say "God is . . ." and proceed to enumerate God’s attributes. To describe the Eternal One in such a sentence is to admit of a division between subject and predicate, in other words, a plurality. (Maimonides writes in Chapter 50 of the Guide, "Those who believe that God is One and that He has many attributes declare the Unity with their lips and assume the plurality in their thoughts.") Therefore, he concludes, one cannot discuss God in terms of positive attributes.
On the other hand, one can describe what God is not. God is not corporeal, does not occupy space, experiences neither generation nor corruption (in the Aristotelian sense of birth, decay, and death). For obvious reasons, Maimonides’ conception of the Supreme Being is usually characterized as "negative theology," that is, defining by the accumulation of negatives. Maimonides writes, "All we understand is the fact that [God] exists, that [God] is a being to whom none of Adonai’s creatures is similar, who has nothing in common with them, who does not include plurality, who is never too feeble to produce other beings and whose relation to the universe is that of a steersman to a boat; and even this is not a real relation, a real simile, but serves only to convey to us the idea that God rules the universe, that it is [God] that gives it duration and preserves its necessary arrangement.