340 NECESSARY TRUTH 



571. The coherent thinking of common sense is founded on 

 these likenesses and differences between the properties of things 

 and on the fact that the properties of all things are conditioned by 

 the properties of other preceding or co-existing things. Thus, 

 owing to some of its properties, a certain object is classified by the 

 postman as a letter, and, owing to others is distinguished from all 

 similar objects as one belonging to a certain person in a certain 

 country, town, street, and house. The postman, the letter, and 

 the person to whom it is addressed, have relations to one another 

 which are conditioned by their respective properties. Now when 

 we speak of cause and effect, what we always mean is that the 

 properties of a thing, or set of things, are, or have been, conditioned 

 (altered or preserved) by the properties of another thing or set of 

 things. Thus, some of the properties of the letter (e.g. its situa- 

 tion) are altered by the postman because he has certain properties 

 (e.g. a desire to perform his duty), and because the person to whom 

 it is addressed has certain properties (e.g. a certain name and 

 situation). The words in the letter are preserved because the 

 paper on which they are inscribed has the property of prolonged 

 persistence. 



572. If we follow any line of thought sufficiently far we are 

 sure in the end to reach limits which mark, apparently, the 

 beginnings of the unknowable and the unthinkable (e.g. the 

 nature of things in themselves, and the terminations, if any, of 

 space and time). Nature is parsimonious and adapts our minds, 

 like our bodies, to practical uses. But within those limits ex- 

 perience stores our memories with data which enable us to reason. 

 All, or nearly all, our reasoning consists in a tracing of cause and 

 effect in a tracing of how the objects we think about are, or have 

 been, or will be, conditioned by their respective properties. At first, 

 especially during infancy, we trace the more obvious relations. 

 But we build continually on the data thus gathered till our reason- 

 ing, tracing effects from many combined causes or from causes that 

 are far remote, grows increasingly complex and subtle. 



573. The limits within which we can think seem to be fixed 

 more by our perceptive than by our thinking powers. We can 

 think only of things that have properties of the same kinds (ex- 

 tension, persistence, colour, etc.) that we have already perceived. 

 Thus, though I can think of a dragon or a demon unlike anything 

 that ever was on land or sea, yet I find I can confer on it only 

 those kinds of properties, however strangely combined and 

 exaggerated, that are already known to me. I am as little capable 



