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.