Science is based on empirical evidence and scientific theories.
Karl popper stated that to accept or to state that a theory is true, there is a certain criterion to be evaluated. You have to evaluate the theory’s falsifiability, or refutability, or testability.
Those evaluations (falsifiability, or refutability, or testability) are the logical possibility that an assertion could be shown false by a particular observation or physical experiment. If a theory is "falsifiable", that does not mean it is false, but it means that if the statement were false, then its falsehood could be demonstrated.
Popper concluded that a hypothesis, proposition, or theory is "scientific" only if it is, among other things, falsifiable. That is, falsifiability is a necessary (but not sufficient) criterion for scientific ideas. Popper asserted that unfalsifiable statements are non-scientific, although not without relevance. For example, meta-physical or religious propositions have cultural or spiritual meaning, and the ancient metaphysical and unfalsifiable idea of the existence of atoms has led to corresponding falsifiable modern theories. A falsifiable theory that has withstood severe scientific testing is said to be corroborated by past experience, though in Popper's view this is not equivalent with confirmation and does not guarantee that the theory is true or even partially true.
The claim "No human lives forever" is not falsifiable since it does not seem possible to prove wrong. In theory, one would have to observe a human living forever to falsify that claim. On the other hand, "All humans live forever" is falsifiable since the presentation of just one dead human could prove the statement wrong (excluding metaphysical assertions about souls, which are not falsifiable). Moreover, a claim may be true and still be falsifiable; if "All humans live forever" were true, we would never actually find a dead