Don't laugh. For centuries, scholars thought that the common housefly had eight legs because Aristotle had said so. It took hundreds of years before someone came along with sufficient initiative to simply impale a fly and by counting its legs learn that it has only six. 

The point is that you can never know whether a claim is true or not unless you check it against the real world. In your head, anything can be true. 

Well, you might say, that was all a long time ago. We certainly don't do that today. 

Oh no? Well, how about these: 

The rich get richer and the poor get poorer.

The single-payer plan is the answer to America's health-care problems.

There's unity in diversity.

America is a racist society.

OK, you might then say, I believe you. But what does all this have to do with me? A great deal. 

You see, if you accept these bits of untested nonsense into your mind, they clog up the channels of clear thinking, and prevent you from making the right choices. 
And so you don't get what you want out of life. 

As Shakespeare put it, “The fault, dear Brutus, is not in our stars, but in ourselves, that we are underlings.” 

Think about it.