At least since Plato’s Theaetetus, philosophers have had a standard definition and understanding of knowledge (although there are a wealth of specifics to argue about within this understanding).

What is knowledge?

Knowledge is truebelief plus a third quality called warrant or justification, or for short “warranted true belief” or “justified true belief.”

Let’s look at these:

1.  Knowledge is always of something true. If P is not true, then I cannot know that P. I can believe that P, because a belief can be either true or false.  If I falsely believe a man is 45 years old, when he is really 47 years old, I do not and cannot know he is 45.

2. Knowledge, subjectively, involves belief, that is, mental affirmation of the truth of something.  It is senseless to say “I know that P … but I don’t believe it!”