"Language acquisition is a subconscious process which happens when people are exposed to language that is comprehensible to them. This means that language is acquired, not learned. People who are trying to learn a language are using a different part of their brains from those who are acquiring it naturally, and are therefore likely to take longer to become proficient."
Harmer, J. (2007). The practice of English language teaching. Pearson Education.

According to Jeremy Harmer, what is the main difference between acquiring a language and learning it?