Once upon a time, we kind of took them for granted. Before the Upheaval we talked about instincts when it came to relationships, or jobs, or that lone guy standing on the corner not moving for thirty minutes.
Because we weren't gifted by nature with usable claws, fangs of any size, the ability to fly, to be able to see in the dark, or any serious boost in hearing or smell, we developed what is loosely termed instincts. In reality, instincts are a collection of signals your brain is getting about your environment. Stuff you aren't really paying attention to, but your subconscious mind is waving its arms and screaming "Pay Attention, Idiot!"
Those of us who learned to trust our instincts survived. When something felt wrong, even though we couldn't see any danger, we stayed away from it. We couldn't explain why, we just did.
For example: Duncan and I were checking out a farm house in eastern Nebraska. By experience, there shouldn't have been any problem. We'd been into a hundred of these places and never had an ounce of trouble. If there was a zombie, between the two of us, it was toast. Duncan had checked the upstairs and came down to see the basement. Right before he opened the door, he stopped. For whatever reason, he wasn't going to open the basement door. When I asked him what the problem was, he just shook his head and said he wasn't going to do it. WE left it alone and went outside. As we rounded the back end of the house, we could see a basement window open. When we approached, several gray arms started reaching out at us. A quick look showed the basement was filled with zombies, and several of them were the little quick kind. If Duncan had opened that door, we'd have been killed. Instincts.
When something in the back of your mind tells you to stop or run, do it. Now. Better to look the fool and be wrong, then be stubbornly dead.
You best weapon is your mind. It pays attention to every detail you miss and puts the picture together into a mural called instincts.
That's why we made it to the top of the food chain.