Thus we shall find Bacon frequently discommending this habit, under the name of 'anticipation of the mind,' and Newton thinks it necessary to say emphatically 'hypotheses non fingo.' It has been constantly urged that the inductions by which sciences are formed must be _cautious_ and _rigorous_; and the various imaginations which passed through Kepler's brain, and to which he has given utterance, have been blamed or pitied, as lamentable instances of an unphilosophical frame of mind. Yet it has appeared in the preceding remarks that hypotheses rightly used are among the helps, far more than the dangers, of science;--that scientific induction is not a 'cautious' or a 'rigorous' process in the sense of _abstaining from_ such suppositions, but in _not adhering_ to them till they are confirmed by fact, and in carefully seeking from facts confirmation or refutation. Kepler's distinctive character was, not that he was peculiarly given to the construction of hypotheses, but that he narrated with extraordinary copiousness and candour the course of his thoughts, his labours, and his feelings. In the minds of most persons, as we have said, the inadmissible suppositions, when rejected, are soon forgotten: and thus the trace of them vanishes from the thoughts, and the successful hypothesis alone holds its place in our memory. But in reality, many other transient suppositions must have been made by all discoverers;--hypotheses which are not afterwards asserted as true systems, but entertained for an instant;--'tentative hypotheses,' as they have been called. Each of these hypotheses is followed by its corresponding train of observations, from which it derives its power of leading to truth. The hypothesis is {83} like the captain, and the observations like the soldiers of an army: while he appears to command them, and in this way to work his own will, he does in fact derive all his power of conquest from their obedience, and becomes helpless and useless if they mutiny. Since the discoverer has thus constantly to work his way onwards by means of hypotheses, false and true, it is highly important for him to possess talents and means for rapidly _testing_ each supposition as it offers itself. In this as in other parts of the work of discovery, success has in general been mainly owing to the native ingenuity and sagacity of the discoverer's mind. Yet some Rules tending to further this object have been delivered by eminent philosophers, and some others may perhaps be suggested. Of these we shall here notice only some of the most general, leaving for a future chapter the consideration of some more limited and detailed processes by which, in certain cases, the discovery of the laws of nature may be materially assisted. SECT. III.--_Tests of Hypotheses._