We do not really need the word force in this sense, since the term relatedness would suffice, and has no misleading associations. But there it is: our business should be to understand clearly what it means. It does not, or should not, I think, mean more, in this connexion, than a particular kind of relatedness in virtue of which an observable kind of change occurs. We may now pass to cause and conditions. When Spencer distinguishes between those conspicuous antecedents which we call the causes and those accompanying antecedents which we call the conditions, he invites the question: What, then, is the essential difference between them? If the accompanying antecedents are distinguished as inconspicuous, we surely need some criterion of the distinction. Furthermore, inconspicuous conditions are, in science, every whit as important as those which are conspicuous. Now we all know that Mill regarded the cause as 'the sum total of the conditions positive and negative taken together.'[45] But he expressly distinguishes between _events_ and _states_.[46] Discussing, for example, the case of a man who eats of a particular dish and dies in consequence, he says: 'The various conditions, except the single one of eating the food, were not events but states possessing more or less of permanency, and might therefore have preceded the effect by an indefinite length of duration, for want of that event which was requisite to complete the required concurrence of conditions.' Again he says: 'When sulphur, charcoal, and nitre are put together in certain proportions and in a certain manner, the effect is, not an explosion, but that the mixture acquires a property by which in given circumstances it will explode. The ingredients of the gunpowder have been brought into a state of preparedness for exploding as soon as the other conditions of an explosion shall have occurred.' And he tells us that physiological processes 'often have for the chief part of their operation to predispose the constitution to some mode of action'.