Each of these modes of proof has its advantages and disadvantages; it is not easy to compare their relative value.  The advantage of positive evidence is, that you have the direct testimony of a witness to the fact to be proved, who, if he speaks the truth, saw it done; and the only question is, whether he is entitled to belief.  The disadvantage is, that the witness may be false and corrupt, and the case may not afford the means of detecting his falsehood.

But in a case of circumstantial evidence where no witness can testify directly to the fact to be proved, you arrive at it by a series of other facts, which by experience we have found so associated with the fact in question, as in the relation of cause and effect, that they lead to a satisfactory and certain conclusion; as when footprints are discovered after a recent snow, it is certain that some animated being has passed over the snow since it fell; and, from the form and number of the footprints, it can be determined with equal certainty, whether it was a man, a bird, or a quadruped.  Circumstantial evidence, therefore, is founded on experience and observed facts and coincidences, establishing a connection between the known and proved facts and the fact sought to be proved.[15]

Under the head of direct evidence, as I shall use the term, would fall the evidence of material objects:  in an accident case, for example, the scar of a wound may be shown to the jury; or where the making of a park is urged on a city government, the city council may be taken out to see the land which it is proposed to take.  Though such evidence is not testimony, it is direct evidence, for it is not based on reasoning and inference.

29.  Direct Evidence.  Direct evidence is the testimony of persons who know about the fact from their own observation:  such is the testimony of the witnesses to a will that they saw the testator sign it, the testimony of an explorer that there are tribes of pygmies in Africa, the testimony of a chemist to the constituents of a given alloy, or of a doctor to the success of a new treatment.  Every day of our lives we are giving and receiving direct evidence; and of this evidence there is great variety in value.

In the first place, no one should place too much reliance on his own casual observations.  It is notorious that we see what we expect to see; and no one who has not deliberately set himself to observe the fact can realize how much of what he thinks is observation is really inference from a small part of the facts before him.  I feel a slight tremor run through the house with a little rattling of the windows, and assume that a train has gone by on the railroad below the hill a hundred yards away:  as a matter of fact it may have been one of the slight earthquake shocks which come every few years in most parts of the world.  The mistakes that most of its make in recognizing people are of the same sort:  from some single feature we reason to an identity that does not exist.