I don't know exactly where I saw this statement, but it says that: You should have the courage to buy at -90% if you expect a 1000% profit in return from something. But I believe that taking wise decisions is much more important than listening to our gut feeling when we trade, as trading should not be based on emotions but practicality. Try to check why it fell down and if you see a reason as well as find that the value is the least it could fall, then buy. And if it's a dump, never get in. It's easy to know because if you see a sudden price boom, then you should not go in even if you know that it may give you something possibly huge because once it starts falling, it doesn't see who is making what.