Although understanding something means we can reason from first principles, discovering these principles requires us to sort them out from the aggregate observations we are built to apprehend.
There must be more than one thing because the ways in which all reality can be made of one thing each require there to be multiple things.
The definition of a whole cannot be found in the definition of its parts, so that things exist does not mean that there is an existence that they are a part of.
The number of kinds of things must be finite since the infinite is unknowable and a finite body cannot be composed of an infinite number of finite bodies.
All principles must be opposites that admit admixtures of opposites, and the properties of an object may consist of combinations of these principles.