300 Mb DE morgan, ON THE GENERAL PRINCIPLES OF WHICH THE 



neither see parts in the whole, nor feel parts in the whole. We are cognisant of more and less, 

 but not by definite junction ; there is no pressure-point of union. That aggregation is possible 

 is a result of experience, even as to pressures in one direction. And as to pressures in different 

 directions we may proceed as follows. Imagine a being in a planet where reasoning beings are 

 fixtures, with the sense of sight, but no sense of feeling; and with the notion of cause and 

 effect, as a consequence, much more nearly* that of mere precedent and consequent than with 

 us. We may even suppose it to be pure speculation with him whether any of the motions he 

 has seen were anything but volitions; he imagining, by a bold stretch of analogy, that moving 

 things may change place by will, just as be himself changes subject of thought by will. If it 

 were put to such a being that the motions he sees were in many cases not volitions of the 

 moving body, but effects of a cause, which beings who have a sense not possessed by him call 

 pressure, and know otherwise than by its effected motion, and if he were asked to investigate 

 the resulting motion when two causes of motion were put in action on the same particle, he 

 might perhaps deliver himself as follows : — " A cause must produce effect, for cause without 

 effect is as inconceivable as effect without cause. As incompatibilities cannot coexist, neither 

 can their causes coexist : for then the efi^ects would also coexist. Now different motions are 

 incompatible : if a particle take one, it does not take the other. Consequently, the notion of 

 the two causes conspiring is absurd. According to your account, these two pressures, as you 

 call them — which you say your superior beings in another planet can comprehend by aid of a 

 sense which I do not posses.s — by merely existing together cease to be themselves, and jointly 

 become something else of the same kind as either. This is incomprehensible.'' In truth, the 

 whole idea of aggregation of two things producing a third of different properties, in which the 

 aggregants are no longer visible or perceptible, seems due to experience: could thought alone 

 predict such a phenomenon ? Even motion itself puts no aggregation in evidence. The ag- 

 gregation of translations was never distinctly before the thoughts until the controversy about 

 the earth's motion brought it into the field in company with aggregation of momenta. 



Granting it known that two pressures must be equivalent to one third pressure, how do 

 we know that there can be but one aggregate.'' Two pressures are applied at the same mo- 

 ment, all difi'erences, except of magnitude and direction, being excluded by hypothesfs. It is 

 also premised that the point of application has no choice and no influence. All we want from 

 experience here is the knowledge that this hypothesis can be realised : the rest follows the 

 notion of causation in its character. To suppose variety of eff^ect possible where every variety 

 of cause has been excluded, is against all the laws under which we inevitably think of causa- 

 tion : these laws it does not concern me to discuss. 



(2) The second postulate is clearly due to experience. In geometrical conclusions it is a 

 law of thought that all parts of space have absolutely the same properties : but by experience 



• Nothing contributes more to our state of thought about 

 causation than our perceptions of cause and efi'ect, as difterent 

 things. We know that a thing which we feel is the cause of a 

 thing which we nee : pressure causes motion. The being 



whom I suppose in the text would need to derive thought 

 about causation from motion producing motion: and would 

 perhaps hardly arrive at habitual thought about cause and 

 effect. 



