Occam's Razor
The simplest explanation that fits the facts is usually the best starting point.
What it means
Occam's Razor is a principle of reasoning that says: when you have competing explanations for something, the one that makes the fewest assumptions is usually preferable. It's named after William of Ockham, a 14th-century friar and philosopher, and it's one of the most useful thinking tools ever devised.
It doesn't say the simplest explanation is always right. It says it's the best place to start. If your car won't start, it's more likely the battery is flat than that someone has secretly rewired the ignition. Both are possible - but one requires far fewer assumptions. Occam's Razor says: check the battery first.
The principle is a guard against overcomplication. We're naturally drawn to elaborate explanations - they feel more interesting, more satisfying, more complete. But complexity has a cost: every additional assumption is another place where you can be wrong. Occam's Razor is a reminder that the most useful explanation is often the one with the fewest moving parts.
In the real world
A friend stops replying to your messages. Your brain starts constructing elaborate theories: they're angry with you, they've been talking about you behind your back, the friendship is over. Occam's Razor suggests a simpler explanation: they're busy, or their phone died, or they simply forgot. The simpler explanation requires fewer assumptions - and it's right far more often than the complicated one.
The thought to hold onto
Before you build a complicated theory, check whether a simple one already explains the facts.