Concise Bible Dictionary:

Scripture abounds with symbols, some parts containing far more than others. Ezekiel, Daniel, and the Revelation are especially full of them. Symbols present, in a concrete form and image, abstract qualities and facts to the mind. In reading scripture, it is necessary to follow carefully the general use of such and such a symbol, throughout the inspired oracles. Thus a lion represents “force;” the sun represents “supreme authority;” eagles, “rapidity of judgment,” and so forth. There is doubtless an abstract idea in each symbol, though it may not always be easy to discover it, and it might vary somewhat in being applied to different subjects. This principle is altogether violated by taking leaven to signify “evil” in the offerings in Leviticus, and what is “good” in the parable of “the leaven hid in the meal” in Matthew and Luke, as is often done.