“Pluralitas non est ponenda sine neccesitate. (Entities should not be multiplied unnecessarily.)”
“Entia non sunt multiplicanda praeter necessitatem.”
“With all things being equal, the simplest explanation tends to be the right one.”
“Keep things simple.”
“All things being equal, the simplest solution tends to be the best one.”
“When you have two competing theories that make exactly the same predictions, the simpler one is the better.”