In an earlier post I went over the four words you need to know to understand Scientology. It'd be a good idea to review that post before reading this one. So, I'll give you a moment to review...
Okay, this post is about the two parts of the mind. I mostly am going over what they are, as it's probably a new concept to a lot of people to have the mind divided up this way. And, if you missed reading it, I cover the Scientology definition of the mind in this post. So, here are the two parts of the mind.
Analytical Mind n. The conscious, aware mind which thinks, observes data, remembers it and resolves problems. It would be essentially the conscious mind as opposed to the unconscious mind. The analytical mind is actually the awareness of awareness unit plus some evaluative circuits (or machinery to make the handling of the body possible). The thetan plus his machine.
Analytical means capable of resolving (problems or situations). The word is from Greek analysis, meaning "resolve, undo, loosen," which is to say, take something to pieces and see what it is made of.
Reactive Mind n. That portion of the mind which files and retains physical pain and painful emotion and seeks to direct the organism solely on a stimulus-response basis. The reactive mind is ruggedly built and never stops operating. Pictures, of a very low order, are taken by this mind of the environment even in some states of unconsciousness. This is the mind that stores up all the bad things that have happened to one and throws them back to him again in moments of emergency or danger so as to dictate his actions along lines which have been considered "safe" before. As there is little thinking involved in this, the courses of action dictated by the reactive mind are often not safe, but highly dangerous. The reactive mind is an unknowing, unwanted series of aberrated (meaning deviating from the rational thought or behavior) computations which bring about a negative effect upon the individual and those around him.
One of the goals in Dianetics is to move the mental recordings of physical pain and unconsciousness that reside in the reactive mind into the analytical mind - so that you can easily remember such recordings without the pain or unconsciousness that has been recorded affecting your decisions or your body negatively. It's all covered in Dianetics: The Modern Science of Mental Health - which is a very cool book. I read it when I was 18 and I think anyone who is curious about their mind and why people act the way they do should check it out.
(Note, these definitions were excerpted from: Dianetics: The Modern Science of Mental Health, Dianetics: The Original Thesis, Science of Survival, Scientology, the Fundamentals of Thought, Scientology 0-8: The Book of Basics and other writings by L. Ron Hubbard.)