If you also take into account the adrenaline dump effects to the mind you might find your mind doing so really strange and bizarre things. The idea is to make your actions instinctive or encode them well to achieve proficient ability yet that is not the crucial matter when we talk of the mind. It is complex and involves our perceptions and beliefs along with the knowledge of what we will "actually encounter" in a real attack. Not the monkey dance which has a path it follows before blows are thrown, generally, but that unexpected and violent attack of some nefarious dude who has his goal and plan done and in place while your mind is trying desperately to understand what is happening, etc.
In a nutshell before you believe or tell yourself the story that what you practice will be there for you make sure you find out all about violence and violent people so you can determine your needs and your beliefs in what you would or would not do if attacked. This is a simplistic answer with only one goal, to get you the reader to find out about all this stuff before you have to deal with it.
If I recommend anything it is to access the No Nonsense Self Defense site by Marc MacYoung then go to the Conflict Communications site by both Marc MacYoung and Rory Miller for a real good start. Then get, read, and re-read books by both these guys. Start with Mr. Miller's new book "Facing Violence" where he succinctly provides the primer to violence before you face it.
We can spout off such things as "mushin and zanshin" but until you actually "know" about the subject and have practiced and trained with that knowledge driving the specifications of the realistic training/practice mushin and zanshin are merely words, sound bites to impress students and the uninitiated.
Don't take my word for it. I do not have the experience these folks have and when I ran my self defense training it was "INCOMPLETE" because I really didn't know or understand what these guys are teaching us.
Oh, as to the word, "only," that is a bit misleading. Fighting whether it is the monkey dance or a true violent encounter involves a lot more than "only the clear mind." There is stuff before a situation, during the situation, and after it is all done. The variables involved begin with the mind and mind training yet they extend far beyond that and if all of the parts are not working together to some extent then it can fail.
The idea is to "NOT FAIL!"