"The only source of knowledge is experience." -Albert Einstein
This quote basically means that you don't know something until you have tried it. You learn from your past experiences. If you made a mistake in your past, you learn from your mistake and you don't make the same mistake twice. One cannot assume something unless they have had the experience of doing that something.
For example, someone who's rich (and always has been) doesn't have the experience of not being wealthy. For another example, a student doesn't have the knowledge of what it means to fail if they have only ever gotten "A's" in their lifetime. The quote by Einstein basically says that you don't know something until you have tried it and that you cannot assume! You learn from past experiences, whether they are good or bad experiences. You can either learn from your bad experiences or relive the good ones. You don't have the knowledge of something until you have experienced it.