Differences in Kind and Differences in Degree
Two things are different in degree when one is more (or less) than the other is some specific respect. One fingernail being longer than another is a difference in degree.
Two things are different in kind when one possess a property the other lacks completely. A rock is different in kind than an orangutan. There are properties an orangutan has that a rock doesn’t like the ability to communicate.
Circles are different in kind than rectangles. Ice is only different in degree than water since the differences between the two can be explained by the rate of motion of their underlying molecules.
Brute animals appear different in kind than plants and humans different in kind than brutes. But appearances, of course, can be deceiving. How do we know?
For a difference to be a difference in kind it must not be something that can be explained by any underlying difference of degree. The question concerning human beings is whether what appears to be radically unique about us — say, our ability to possess concepts, reason via modus ponens, etc. — can be explained by underlying degrees of brain complexity.