No more things should be presumed to exist than are absolutely necessary.
For nothing ought to be posited without a reason given, unless it is self-evident (literally, known through itself) or known by experience or proved by the authority of Sacred Scripture.
It is vain to do with more what can be done with less.
With all things being equal, the simplest explanation tends to be the right one.
The explanation requiring the fewest assumptions is most likely to be correct.
When you have two competing theories that make exactly the same predictions, the simpler one is the better.