3) I am choosing a major in philosophy and a minor in religious studies because it is the easiest way to keep a high GPA without working too hard.
4) If all B is C and all A is B then all A is C.
Sentence (4) is the sort of formal inference that gladdens the heart of logicians. Its validity can be explained extensionally without inquiring into the meanings of 'A', 'B', or 'C', provided they are class terms. The other three sentences involve material inferences. Their validity depends both on the meanings of the terms used, on various word-world connections, and on presuppositions about reality. When we make such inferences we reason in terms of things, their properties, characteristic activities and relations: the proportions of gasoline and air required for combustion; the normal reactions of cats; the undemanding nature of many philosophy, and most religious studies, courses. Particular presuppositions may be doubted or denied. But it is not possible to carry on normal inferences while systematically doubting the conceptual network that supports it. Nor is it possible to carry on normal discourse while treating the nested presuppositions involved as hypotheses with different probabilities.
LCP presupposes a large, but not so amorphous, body of claims accepted as true. One could, for example, use the Handbook of Chemistry and Physics to formulate an indefinite number of claims like” The density of magnesium is 1.75 g/cm2"; or "The coefficient of linear expansion of aluminum is 26 x 10-6/ °C". Experimenters routinely rely on the truth of such claims. If a particular statement is doubtful then one checks it by getting better sources or making measurements that are more accurate. One can not cast doubt on all of them simultaneously and carry on the practice of physics. For the most part they are accepted as facts about the world, not matters of inference. From a pragmatic perspective such claims are objective in the sense that there is no further court of appeal. If a particular claim is doubted, then it is checked by doing better physics, not by going beyond physics.
This pragmatic justification, however, does not justify claims to the effect that basic scientific statements are true because they correspond to reality as it exist independent of human knowledge. Both ordinary and scientific claims fit reality as categorized. Claims depend on presuppositions. Presuppositions ultimately depend on a categorial framework. The further we recede from surface claims through presuppositions to the categorial core of language, the less significance there is to assertions of a correspondence with objective reality. What does it mean to claim that a categorial system corresponds to reality?
For a concrete illustration consider a relatively simple experiment that involves extending classical concepts into the quantum domain. In 1914 James Franck and Gustav Hertz performed an experiment, schematized in Fig. 1, in which electrons are emitted from the thin part, D of a current-carrying platinum wire. N is a fine cylindrical platinum wire mesh with a 4-cm. radius surrounding D, and G is a cylindrical platinum foil.31 The evacuated tube, G, contains mercury vapor at very low pressure. Positive voltage on N accelerates electrons from D to N and then decelerates them on the much shorter trajectory from N to G. As the voltage on N was increased the kinetic energy of electrons striking G increased. When the voltage at N reached 5 volts the number of electrons reaching G dropped to zero. Franck and Hertz interpreted this as the initiation of ionization. Low energy electrons have elastic collisions with the ambient mercury atoms. When an electron
31 A translation of the original text is in Boorse and Motz (1966), pp. 766-788).