In ordinary usage the word “evidence” is pretty vague, and means anything that will help to establish one side or another of any question, whether of fact or of policy.  The word, however, comes ultimately from the law, where it is used for the testimony, either oral or written or material, which is brought in to establish the truth of assertions about fact:  evidence is set before the jury, which under the common law decides questions of fact.  In almost any argument of policy, however, we use facts as reasons for or against the policy which is in question, and therefore inmost cases we must use evidence to establish these facts; in many cases, when the facts are established there is no further disagreement about the policy.  For example, in arguments for and against state prohibition of the liquor trade, it is an essential fact to determine whether in status where prohibition has been tried it has failed or succeeded, and another essential fact whether under similar conditions a combination of high license and local option has or has not produced less drunkenness.  Both are extremely complicated and difficult facts to decide; but if clear evidence can be brought forward to establish them, reasonable-minded people would generally hold as settled the question of the policy which should be adopted.  Similarly, an argument for the popular election of senators would undoubtedly make large use of the alleged fact that, in elections by the legislatures, there has been much undue interference by special interests and rich corporations; and the assertion of this fact would have to be supported by evidence.  If this fact were thus clearly established, it would be recognized as a strong reason for a change in our present policy.  In the interest of clearness of thought it is worth while to remember this distinction; for, as we shall see, it is only by so doing that we can determine when the ordinary rules of logic do and when they do not apply to the processes of reasoning on which argument is based.  I shall speak here, therefore, of the evidence for facts, and of the reasons for or against a policy.

It may be said in passing that the highly complicated rules of evidence at the common law have practically nothing to do with our present subject, for they spring from very special conditions, and have been molded by very special purposes.  Their object is to establish, so far as is possible, principles which will apply to all cases of a like nature; and they therefore rule out many facts and much evidence which outside the court we all use without hesitation in making up our minds.  The jury system has had a curious and interesting history:  and judges have built up hedges around juries which seem to the layman merely technical, and unnecessary for the ends of justice.[14] Yet though the sweeping away of many of these rules from time to time shows that there has been and perhaps still is justice in this view, one must remember that the whole common law is based on the application of