Conjecture is probable truth. Some subjects only furnish a sufficient number of facts to make them probable in the lowest degree—not to decide them as positively true. The propositions expressing results pertaining to such subjects are called conjectures.
A conjecture founded on no fact or upon too few to make it likely, is called a vagary.
It will be seen that probability is a thing of degree. A probability may vary in weight from a moral certainty, where it ranks next to a physical certainty, down to a conjecture, and descend lower in likelihood till it is lost in conjecture.
Lord Kames remarks, in his preface to his 'Sketches'—'Most of the subjects handled in the following sheets, admit but of probable reasoning: and, with respect to such reasoning, it is often difficult to say, what degree of conviction they ought to produce. It is easy to form plausible arguments; but to form such as can stand the test of time, is not always easy. I could amuse the reader with numerous examples of conjectural arguments, which, fair at a distant view, vanish like a cloud on a near approach'. Did all authors so judiciously apprise their readers of the probable logical value of their speculations, fewer would be misled than now.
To numerous questions of undoubted interest, which have been agitated in all ages, only a moderate degree of certainty attaches—these are termed speculative. Such subjects may afford but few facts and instances, and the chances of conclusiveness may seem remote—yet ultimate results are not to be despaired of: the new comparison of conjectures and the arrangement of facts daily throws new light on age-contested points. Systems of conduct should not be founded on conjectures in opposition to evident moral utility; but if speculation is kept 'within the sphere of speculation, it may be prosecuted with safety and prospect of success.
There are problems in metaphysics as there are in mathematics, which may be demonstrated to be insolvable. To describe the limit of human power with respect to contested questions will yet result from speculative controversy. The capacities of our understanding will be one day well considered, the extent of our knowledge discovered, and the horizon found which sets bounds between the enlightened and the dark part of things—between what is and what is not comprehensible by us. But this will only be when the untried has been universally attempted in all directions. Bailey, I think, has defined truth as being that which is universally accepted after having been universally examined. Little of this truth is yet extant. When every man shall be a thinker, when the autobiography of intellect shall be more freely furnished than it ever yet has been, unanimity of opinion not yet dreamed of will prevail. Harmony of opinion is the sign of intellectual conquest—the standard-bearer of truth no advocacy is victorious while dissent occupies the field.
What we know to be true, is knowledge; what we have only reason to believe true, is opinion. All human information is made up of knowledge and opinion. The primary importance of knowledge is evident from the fact that knowledge is the umpire of all opinion. We believe in the existence of the ruins of Palmyra and Thebes, and in certain discoveries of algebraists and astronomers. It is our opinion that these things are true, although we may never have visited Palmyra or Thebes, nor made the calculations of the algebraist, nor the observations of the astronomer. In these cases our belief is founded on our experience and knowledge of mankind. It is quite true that travellers exaggerate, and scientific men are sometimes mistaken; but we know that there is always some truth at the bottom of what is communicated by well-meaning writers. More or less, every man's experience assures him of this; and it is the cause of our reliance on the records of history, and the reports of science. Therefore, since all information is made up of knowledge and opinion, plainly knowledge is the one thing which comprises all intelligence.
'Questions of fact,' observes Pascal, in his celebrated 'Provincial Letters,' 'are only to be determined by the senses. If what you assert be true, prove it to be so; if it be not, you labour in vain to induce belief. All the authority in the world cannot enforce or alter belief as to facts; nothing can possibly have power to cause that not to be which actually is.'*
A remarkable instance of the verification of what was assumed to be is related of Pascal by Goodrich. 'Pascal was a philosopher even in childhood. At a very early age he was taught the ten commandments. For several days after, he was observed to be measuring the growth of a blade of grass. When asked the meaning of this, he replied, "The fourth commandment says, 'Six days shalt thou labour, but the seventh is the Sabbath in which thou shalt do no work.' Now I wished to ascertain if nature obeyed this great law, and therefore measured the grass, to see if it grew as much on Sunday as on other days." '**
'We are informed,' says Beattie, 'by Father Malebranche, that the senses were at first as honest faculties as one could desire to be endued with, till after they were debauched by original sin; an adventure from which they contracted such an invincible propensity to cheating, that they are now continually lying in wait to deceive us. But there is in man, it seems, a certain clear-sighted, stout, old faculty, called reason, which, without being deceived by appearances, keeps an eye upon the rogues, and often proves too cunning for them.'***