Alan Jacobs clearly explains how analogies work in this post.
Very few people understand how to evaluate analogies properly. An analogy will have explanatory value if the things or experiences or events or ideas likened to one another are indeed alike in the respect called attention to by the analogy. Far too many people think they can deny the validity of an analogy between X and Y by pointing out ways in which X and Y are different. Yes, and if they were not different you couldn’t analogize them because they would be the same thing. In Thomistic terms, you do not discredit an exercise in analogical predication by gleefully announcing that the predication is not univocal.The use of analogies can be for clarity, emphasis, or rhetorical purposes. People may show poor judgment in their choice of analogy, but one's disapproval of a component of the analogy does not invalidate it.
Here’s the proper way to evaluate an analogy:
- Ask this question: Does the person making the analogy between X and Y explain the respect in which he or she claims that X and Y are similar?
- If not, ask the person to clarify that point.
- If so, think about whether X and Y are indeed similar in the respect specified. If so, the analogy is legitimate. If not, the analogy fails.
- Feel free at this point to pursue other questions about the analogy, e.g., whether even if legitimate it identifies an important similarity, or whether the analogy does the intellectual work its maker thinks it does.