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Similar in relationship to each other as perspicuity and perspicacity (see Dec. 30, 2009 post) are imply and infer.

Imply is what the communicator does, infer is what the communicatee does.

Communicatee is not a word in my dictionary, but it ought to be. Perhaps someone can suggest a good word in its place: communicant is a religious term and would be close, and "person with whom the communicator is communicating" is too long, so I'm going with communicatee. We have speaker and hearer, we have sender and recipient, but what do we have for the broader communicator?

Anyway, imply has a very old etymology. It comes to us from Middle English through Old French and Latin and before that from Indo-European. Im means in and ply comes from (we think) the Indo-European word plek, which means to plait or wrap together. It is where the Greek word plekein comes from, which means to braid. The idea is that of wrapping into the meaning of the communication an unexpressed thought that requires untangling to understand. It is not the direct indication of the communication, but something hinted at or alluded to.

Infer is when someone assumes and unwraps a meaning from a communication. (It may or may not have been implied.) It comes from the Latin word inferre which means to carry in. When you infer you're carrying in to the communication something that you think was woven in to the previous communication.

My dictionary (Webster's New World Dictionary of the American Language) provides good explanation of some similar words and further explains the differences:

"Infer suggests the arriving at a decision or opinion by reasoning from known facts or evidence...; deduce, in strict discrimination, implies inference [there's a confluence of our two previous words!] from a general principle by logical reason...; conclude strictly implies an inference that is the final logical result in a process of reasoning."

Conclude and deduce do not refer to communication necessarily, but infer does. In its best usage infer doesn't apply to most of what Sherlock Holmes did but deduce and conclude both do. You can come to a conclusion through deduction that is observation but can only infer from a communication.

Clear as mud?