Angels are natural forces

 Moreh Nevuchim (2:06.03) Say to a person who is believed to belong to the wise men of Israel that the Almighty sends His angel to enter the womb of a woman and to form there the fœtus, he will be satisfied with the account; he will believe it, and even find in it a description of the greatness of God's might and wisdom; although he believes that the angel consists of burning fire, and is as big as a third part of the Universe, yet he considers it possible as a divine miracle. But tell him that God gave the seed a formative power which produces and shapes the limbs, and that this power is called "angel," or that all forms are the result of the influence of the Active Intellect, and that the latter is the angel, the Prince of the world, frequently mentioned by our Sages, and he will turn away; because he cannot comprehend the true greatness and power of creating forces that act in a body without being perceived by our senses.

Moreh Nevuchim (2:06.03) Our Sages have already stated--for him who has understanding--that all forces that reside in a body are angels, much more the forces that are active in the Universe. The theory that each force acts only in one particular way, is expressed in Bereshit Rabba (chap. 1.) as follows: "One angel does not perform two things, and two angels do not perform one thing"; this is exactly the property of all forces. We may find a confirmation of the opinion that the natural and psychical forces of an individual are called angels in a statement of our Sages which is frequently quoted, and occurs originally in Bereshit Rabba (chap. lxxviii.): "Every day God creates a legion of angels; they sing before Him, and disappear." When, in opposition to this statement, other statements were quoted to the effect that angels are eternal--and, in fact, it has repeatedly been shown that they live permanently--the reply has been given that some angels live permanently, others perish; and this is really the case; for individual forces are transient, whilst the genera are permanent and imperishable. 

Moreh Nevuchim (2:06.03)Again, we read (in Bereshit Rabba, chap. lxxxv.), in reference to the relation between Judah and Tamar: "R. Jochanan said that Judah was about to pass by without noticing Tamar, but God caused the angel of lust, i.e., the libidinous disposition, to present himself to him." Man's disposition is here called an angel. Likewise we frequently meet with the phrase "the angel set over a certain thing.'' In Midrash-Koheleth (on Eccles. 10:7) the following passage occurs: "When man sleeps, his soul speaks to the angel, the angel to the cherub." The intelligent reader will find here a clear statement that man's imaginative faculty is also called "angel," and that "cherub" is used for man's intellectual faculty. How beautiful must this appear to him who understands it; how absurd to the ignorant!