 An additional mistake which Popper discerns in historicism is the failure of the historicist to distinguish between scientific laws and trends. This makes him think it possible to explain change by discovering trends running through past history, and to anticipate and predict future occurrences on the basis of such observations. Here Popper points out that there is a critical difference between a trend and a scientific law: the latter is universal in form, while a trend can be expressed only as a singular existential statement. This logical difference is crucial: neither conditional nor unconditional predictions can be based upon trends, because trends may change or be reversed with a change in the conditions which gave rise to them in the first instance. As Popper puts it, there can be no doubt that