In short, the opposite of logic.
It is the fundamental difference between men and women that makes understanding one another impossible. Men are born with logic, women are born with anti-logic; these are immutable and unchangeable differences. A man can not be born with anti-logic, nor can he acquire it at anytime in his life. A woman can not be born with logic, nor can she ever hope to acquire it at anytime in her life.
It is not possible for any individual to comprehend someone of the opposite logic. Many have tried and some devote their lives and professions to this impossible notion; they become very bitter and unhappy people.
Like matter and anti-matter, logic and anti-logic are contrary to the other. If the two were to merge into one, a black hole will form and destroy life on Earth. Luckily, like matter and anti-matter, logic overwhelmingly dominates anti-logic(99% to 1%), this is why humans have become as civilized as we have, else we wouldn't have made it past the first stage of our sentient evolution.
Often people are confronted with a member of the opposite logic and an argument ensues, inevitably ending the same way it started, in disagreement. But now you know why, so next time, it is the responsibility of the man to stop the woman from talking and explain to her that she is using anti-logic, a form of logic that has no merit in an argument. She won't understand of course but that is to be expected.
For an example of how logic and anti-logic is manifested between a man and a woman, you need not look any further than the first man and woman on Earth, Adam and Eve.
God gave them everything anyone could ever want; perfection. However there was an apple tree that God emphasized to Adam and Eve not to eat from, for it will surely screw everything up. Adam, (a person of logic), understands God perfectly ("Don't eat bad apple from tree = Good"..."Eat bad apple from tree = Bad") Eve however, (a person of anti-logic) had her own interpretation ("Don't eat bad apple from tree = Bad"..."Eat bad apple from tree = Good")
So Eve joyously skipped and sang her way to the tree, plucked and apple, and took a bite. And that's where man began to need many other new words such as: anger, stress, resent, depression, hopelessness, and many many more.
REMEMBER - It is futile for any man to understand a woman, and futile for any woman to make sense.