Pegusi and Unicorns.

There are four levels of atheism:

Level 0. These are what some people call "believers". They think that an invisible, undetectable monster that made everything, knows everything and can do anything is real, though they have no tangible evidence for such a being.

Level 1. These people are usually called agnostics.They do not know whether or not the monster exists.

Level2. As there is no evidence for the monster these people believe that it does not exist but do not say that it would be impossible.. They are "unicorn" atheists.
Unicorns are plausible. Consider the narwhal, a one horned mammal, deer and cattle which are ungulates with horns and a horse, shaped very much like a unicorn - so it is not impossible for a unicorn like mammal to exist; it would be anatomically feasible but now very unlikely given the path that evolution has taken. In the same way, Level 2 atheists claim that gods do not exist but they do not have the knowledge and understanding to know that they could not.

Level3. These are "pegasi" atheists. They know not only that gods do not exist but that they would be impossible. The universe would not tolerate such entities at odds with the rest of the cosmos. In the same way the Pegasus, a horse shaped mammal with wings that can fly in an earth atmosphere under earth gravity does not and could not exist.

After 50 years studying the sciences I regard myself as a third level atheist.

There are many sub categories within these levels such as "weak and strong, positive and negative" and Professor Dawkins 7 point scale. They are just as valid and will all fit under the headings I have just outlined.