What is the Law of Falsifiability?

Some assertions (theories) are falsifiable, and others are not. The concept is that if a proposition can be shown to be false by observation or experiment, then it is falsifiable. This borders on being a circular definition. It might be clearer and more accurate to express it this way. If an assertion is false, then it can be demonstrated to be false by observation or experiment. One subtle thing to keep in mind: it is not correct to say that ONLY assertions that have been proven wrongare falsifiable. During the course of any experimental project, any assertions, hypotheses or theories should be falsifiable, in the sense that the assertions, hypotheses or theories can be reasonably held up to scrutiny by some special form of observation or by the manipulation of variables in an experimental setting. Here is a theory: Caffeine interferes with sleep. This theory is falsifiable, even though it happens to be true as far as I am aware. IF THIS THEORY IS FALSE, then by asking the right questions and doing careful enough experiments, then it can be demonstrated to be false. For example, people may say that it is not caffeine, but some other compound in coffee, tea, chocolate, etc, that interferes with sleep. So we can test this and see what results. Or, many people can be given Caffeine under controlled conditions so that sleep patterns can be carefully examined. Here is another: Being exposed to full-spectrum light all evening before going to bed for the night at 11pm will enhance and deepen your normal sleep patterns. You may or may not know whether this assertion is true! If you do not know whether or not it is true, you can still grasp that it would be possible to demonstrate that it is false, if it is false. Here is another: Whenever you have trouble sleeping, it is because someone is working a spell on you. Is this assertion falsifiable?