"reality." It is content if the impressions received are consistent (that is, reproducible and uncontradictory) and coherent (that is, capable of being related one to another). Science therefore excludes from its scope all knowledge obtained from other than sensory impressions (from intuition or revelation, for example) unless such knowledge is capable of being tested in the ordinary way-when the intuition becomes a working hypothesis. Recorded observations constitute the data from which generalizations are drawn, concerning which assumptions are made and around which theories are constructed. The important thing about science is its impartial and critical attitude to its own data which demands the checking of observations by diierent observers. The individual can only be scientific when he is one of a group. While a science may be born out of purely observational data, history shows that the fullest and most fruitful development has been attained only when both observational and experimental data are available. Theories serve a threefold purpose. In constrncting them the aim is always to link together generalizations that are as far apart as possible. The more farreaching a theory is the more useful it is. In this way theories weld science into a compact whole. Again, a good theory in addition to linking generalizations that are already known will usually suggest hitherto unsuspected generalizations. The search,for data to prove or disprove these predictions serves both to enlarge the general body of knowledge and to test the usefulness of assumptions embodied in the theory. The great difference between a scientific theory and a pseudo-scientific one is this: that a scientific theory stands or fallsby the correctness of its predictions, while a pseudo-scientific theory is upheld because its logic is sound or its assumptions plausible. Finally, by providing links between generalizations, theories often enable us to bridge the gap between a field where experiment is possible and vne where only observational data are available. This is beautifully illustrated by modern astrophysics. Perhaps some day economics will advance to this stage. Let us now see what practical value there is in the foregoing analysis. Fist, it emphasizes the nature of scientific data and the attitude of science toward its data. Second, i t is a reminder that generalizations are founded in data and derived from data. They are tested by gathering more data. They can only be attacked by adducing conflicting data. The question of what "ought" to be does not enter, i t is purely and simply a matter of what "is."
Third, it shows that a theory combines the three elements, generalizations, hypotheses, and logic. Therefore, any theory can be examined as to the soundness of these three ingredients, but the criteria of their soundness are quite different. A generalization is justified or discredited by data. Logic consists of agreed rules for reasoning. Hypotheses are justified by, first (and least important) their success in correlating generalizations, and second (and critically important) their success in leading to verifiable predictions. A theory that only lets us be wise after the event is useless. It is not necessary that a theory should predict everything-but what predictions a theory does make must be correct. This criterion of utility is the only criterion of an acceptable hypothesis. Plausibility is not important. (Compare relativity and wave mechanics.) Bound up with the criterion of utility is the question of simplicity. That theory is the most useful which is the simplest, which means capable of linking the most generalizations with the help of the fewest assumptions. The criterion of an hypothesis is its usefulness The criterion of a simplifying assumption is its fairness and convenience. The criterion of a restriction is its applicability in a particular case. In conclusion, we may add a word concerning definitions. A definition is a form of words stating the meening of a word. There are two sorts of wofds to be defined: the words used in describing phenomena are of one kind; some of the words used in framing hypotheses are of the other. A word used in describing phenomena must be defined in such a way as to enable +ny person possessed of the necessary skill and facilities to recognize the thing defined. Every technical word used in a generalization must be de6ned in this way. The word "element" is sometimes defined as indicating a substance composed of one kind of atom only. As a definition this is worthless-unless i t be taken as defining the word "atom!" The word element' should rather be defined somewhat as follows (at any rate for chemical purposes). An element is a pure substance (previously defined) which cannot he separated into parts having diierent chemical properties (previously defined) by the use of ordinary physical and chemical methods. The second kind of word is'that used to designate something hypothetical. Atoms, molecules, valence bonds, electrons, and so forth, are whatever we choose to say they are, they have whatever properties it may be useful to assign to them. Definitions of words of this kind may be altered almost a t will. Definitions of words of the other kind can only he altered if it becomes evident that the previous definition was incomplete or ambiguous.
PITTSBURGH SUMMER ANNOUNCEMENTS Professor Wojciech Swietoslawski, formerly Head of the Physico-Chemical Institute in the University of Warsaw and the present Chairman of the Committee on Physico-Chemical Stand-
a d s of the International Union of Chemistry, will offer lectures in the Department of Chemistry, University of Pittsburgh, on calorimetry and ehulliometry, from July 2nd to July 30th.
