120 THE GENESIS OF SCIENCE. 



edge ; but between the successive phases of science itself, 

 or knowledge itself — whichever we choose to call it. In 

 its earlier phases science attains only to certainty of fore- 

 knowledge ; in its later phases it further attains to com- 

 pleteness. We begin by discovering a relation : we end 

 by discovering the relation. Our first achievement is to 

 foretell the kind of phenomenon which will occur under 

 specific conditions : our last achievement is to foretell not 

 only the kind but the amount. Or, to reduce the proposi 

 tion to its most definite form — undeveloped science is qual- 

 itative prevision : develoj^ed science is qiiantitative previ 

 .. sjon. 



This will at once be perceived to express the remaining 

 distinction between the lower and the higher stages of posi- 

 tive knowledge. The prediction that a piece of lead will 

 take more force to lift it than a piece of wood of equal size, 

 exhibits certainty, but not completeness, of foresight. The 

 kind of effect in which the one body will exceed the other 

 is foreseen ; but not the amount by which it will exceed. 

 There is qualitative prevision only. On the other hand, the 

 prediction that at a stated time two particular planets will 

 be in conjunction ; that by means of a lever having arras in 

 a given ratio, a known force will raise just so many pounds ; 

 that to decompose a specified quantity of sulphate of iron 

 by carbonate of soda will require so many grains — these 

 predictions exhibit foreknowledge, not only of the nature 

 of the effects to be produced, but of the magnitude, either 

 of the effects themselves, of the agencies producing them, 

 or of the distance in time oj space at which they will be 

 produced. There is not only qualitative but quantitative 

 prevision. 



And this is the unexpressed difference which leads us 

 f to consider certain orders of knowledge as especially scien- 

 \ lific when contrasted with knowledge in general. Are the 

 \ phenomena measurable f is the test which we unconsciously 



