Popper's idea to solve this problem is that while it is impossible to verify that every swan is white, finding a single black swan shows that "not" every swan is white. We might tentatively accept the proposal that every swan is white, while looking out for examples of non-white swans that would show our conjecture to be false. Falsification uses the valid inference "modus tollens": if from a statement formula_1 (say some law with some initial condition) we logically deduce but what is observed is we infer that formula_1 is false. For example, given the statement "all swans are white" and the initial condition "there is a swan here", we can deduce "the swan here is white", but if what is observed is "the swan here is not white" (say black), then "all swans are white" is false, or it was not a swan.