Hallucinations are possible, and in the more abstract domains of mental activity errors are possible also; and will be ever more frequent. Nevertheless the reality of outside facts in the sense stated above can as little be doubted as the reality of immediate perception; and all the facts established by science, if they are but true, are as much realities as is the resistance of the table to the pressure of my hand or the perception of the sheet of paper by my eye. #Inferential facts, if true, are real.# Facts established by science are those observations which are made with all the necessary exactness as well as completeness from certain groups of experiences, and formulated with precision. The theory of atoms, for instance, is true in so far as all elements combine in certain proportions, which shows that the ultimate particles of which the elements consist are of a definite mass. Atoms, if the word is understood in this sense, are realities. The theory of atoms, however, is not proved in the sense that atoms are ἄτομοι; or single, isolated, minute bodies of a peculiar individuality—separate, indivisible, and eternal entities. Whether they are concrete things or certain forms of motion in a continuous substance, whether they are vortices or whirls of a certain density and velocity in an ether ocean, or whatever else be their character, is not yet known. If we exclude from the concept "atoms" all hypothetical views and confine their meaning strictly to the formulation of certain experiences, we have to deal with facts that are real. Theories are true in so far as they comprehend in a formula a certain group of facts, and a hypothesis becomes reliable to the extent that it agrees with facts. The slightest actual disagreement with facts is sufficient to overthrow the most ingenious hypothesis. * * * * * This leads us to the question, What is meant by _true_? What is truth? #Facts and reality. Truth and mind.#