beings, they are sometimes mistaken. Then we want evidence that is more 

 reliable than goodwill or sincerity. What kind of evidence do we want? 

 What kind is possible? 



We can know about the past, and especially about events that occurred 

 before any human being could report or record them, only by interpreting 

 significant facts. But the only facts we have are about present conditions in 

 the world and about processes now going on. What can present facts tell us 

 about the past? 



The facts themselves tell us nothing. They take on meaning only as we 

 ourselves make up our minds as to how things come to happen. If we make 

 certain assumptions about the workings of the world, the facts tell us one 

 thing; if we make other assumptions, the same facts tell us something quite 

 different. We know, for example, that the farther we dig into the earth, the 

 hotter it gets. One person concludes that when the earth was made, the core 

 was made hot and the crust cold. Another, from the same "facts", declares 

 that something is happening inside the earth to generate heat. A third might 

 say, "The earth must have been very hot at one time, and it hasn't completely 

 cooled yet". Or take the fact that brooks and rivers wear away soil and rocks, 

 which later settle to the bottoms of lakes and ponds and oceans; or the fact 

 that rocks are found in layers of different thicknesses, and at various angles. 

 One person says, "When the earth was made, parts of it were laid in hori- 

 zontal layers and other parts were made with layers slanting at various angles". 

 Another person might say, "In the course of time sediment became hardened 

 into rock; some layers took much longer to form than others; the character 

 of the sediment varied from time to time; something must have pushed the 

 horizontal layers out of place". 



Choice of Assumptions We all make assumptions about the nature of 

 the world, about why things happen as they do. But we do not all make the 

 same assumptions. In these imagined cases one observer seems to assume that 

 "the world was made" once and for all and has remained as it is from the be- 

 ginning. Another seems to assume that what we see today is the present state 

 in a long process, that what is has come naturally out of what was. We are 

 apparently free to assume, or "believe", whatever we wish. But the choice 

 we make is not entirely a matter of taste or of religion. For our assumptions 

 turn out to be of great practical importance. 



In all practical studies — agriculture and engineering, medicine and states- 

 manship, business and housekeeping — three sets of problems have to be solved: 

 (1) How can we cause desirable changes to taJ^e place? (2) How can we prevent 

 undesirable changes from talking place? (3) How can we best meet unavoidable 

 changes? 



To solve such problems, however, we must first settle the "theoretical" 

 question How do things wor)(? What are we to assume about the world? We 



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