A car suddenly refuses to start and the fuel gauge points to empty. Suspecting an empty tank is usually more reasonable than proposing, all at once, that the tank is empty, the fuel pump has failed and the wiring is broken. This preference against idle assumptions is commonly called Ockham's razor. It is associated with the fourteenth-century philosopher William of Ockham, although the familiar slogan about not multiplying entities beyond necessity is not a sentence he left us, and related principles of economy predate him.
The crucial qualification is ‘other things being equal’. If two explanations fit the evidence equally well and account for the same phenomenon, an extra assumption that adds no predictive or explanatory work has no initial advantage. Simplicity here might mean fewer causes, entities or adjustable parameters, not merely a shorter sentence.
Simplicity is not proof of truth, however. The gauge could be faulty, and the engine might indeed have a second problem. A more complex account cannot be discarded merely for being complex if it explains additional evidence. What counts as simpler can also depend on how theories are described, so simplicity alone cannot settle every comparison.
My view is that Ockham's razor works best as a discipline for inquiry: do not invent additional causes before the evidence requires them, and test the explanation with fewer assumptions first when that test is cheaper. It tells us where to begin investigating, not that reality must be simple. When new evidence arrives, the more complex explanation may win.
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