ON THE NATURE OF LIFE 211 



be seen to be consequences of the assumptions that we have just 

 made. 



We start with certain very simple concepts, mass and time 

 and space and the law of gravitation, which is a relation between 

 mass and space and time. Given these concepts and a know- 

 ledge of the velocities of the various bodies, and we can then 

 deduce all their movements, present, past, and future. Every- 

 thing that happens in the motions of the solar system happens 

 because the various bodies have mass and attract each other in 

 a certain way. Therefore we " explain " these very complicated 

 movements by the concepts of mass and gravitation. The latter 

 are simple or irreducible that is, we cannot (so far) explain them 

 by supposing them to be the consequences of something still 

 simpler and more general than mass and gravity. 



Knowing the positions of the planetary bodies at any moment, 

 as well as their velocities, knowing also their masses and assum- 

 ing that the law of gravitation holds good in all circumstances, 

 we can then find what will be their positions at any future time 

 and what were their positions at any past time. These pre- 

 dictions and retrospects generally are successful, and when they 

 fail (as they do in rare instances) astronomers assume other simple 

 concepts (as in the theory of relativity), and then their calcula- 

 tions work out true to what can be observed. Thus we have a 

 planetary theory, which is the explanation of a host of complex 

 events by a very simple hypothesis that the cosmic bodies have / 

 mass and attract each other. This theory is verified, inasmuch 

 as it enables us successfully to predict forwards or backwards. 



Now if we had a theory of life we should also have certain very 

 simple, irreducible concepts, and it would be the case that all 

 organic phenomena growth, reproduction, assimilation, ex- 

 cretion, behaviour, adaptation, evolution, and so on would be 

 the inevitable consequences of these fundamental concepts, just 

 as solar and lunar eclipses, tides, seasons, etc., are the inevitable 

 consequences of the ways in which the heavenly bodies move, 

 which are, again, the consequences of the law of gravitation. 

 Knowing these fundamental factors, we should be able to 

 " explain " life. We should be able to say what an animal will 

 do at any future time, and what it did at any past time. We 

 ought to be able to trace out the past history of the species to 

 which it belongs, as well as its future evolutionary history, just 

 because we know what the animal now is and what are the 

 concepts by means of which we " explain " its activities. 



