302 History of Philosophy. 



produced on the mind by external objects, and the traces of which 

 are preserved by the memory. We now come to the next faculty, 

 which consists in the power of comparing the divers classes of im- 

 pressions stored up in the memory, and thence judging of the 

 analogies or distinctions existing between them. An infant will cry 

 because it cannot reach the moon a mountaineer in a desert or a 

 landsman at sea will form false estimates of distance. These errors 

 of judgment can only be corrected by experience, that is, by collect- 

 ing in the memory a series of examples of similar sensations, and by 

 comparison judging of- the due value to be allowed to each. This 

 faculty we will name understanding. By it we are enabled to, form 

 notions of size and relation, and are able to abstract accidental cir- 

 cumstances of time and space from the images received by our 

 senses. These present to us notions in a confused or concrete state, 

 while by the understanding we are enabled to survey them in an 

 abstract or clear and unencumbered state. 



The combinations formed by the understanding enable us to form 

 correct judgments of all things that exist, and will lead us to a just 

 and extensive acquaintance with all the objects in the material world, 

 or with those subjects which are treated of in what are commonly 

 called the exact sciences. But these by no means form the limit of 

 human intelligence. There is a sort of general notions not furnished 

 by the perception of material objects, but derived from a totally 

 different source. These Plato calls IDEAS, but the doctrine connected 

 with them is so imperfectly developed in his writings, either from 

 the difficulty of the subject or the wish to attach a mystery to the 

 most abstruse portions of his science, that the greatest difficulty has 

 been experienced by all (even the most competent) who have studied 

 his philosophy in forming an intelligible explanation of his meaning. 



IDEAS, he says, are the eternal and immutable forms, the nature, 

 the essence of things. They have not been produced by impressions 

 from external objects, and have therefore a peculiar existence and 

 independent value. They are unlimited by any conditions of time, 

 space, or form ; they are general notions of the highest order. Not 

 being derived like all other notions, they do not correspond with any 

 form of matter, and are independent of experience, or innate. 

 Through them we become acquainted with that which is possible or 

 which ought to exist, as the lower faculties of mind enable us to form 

 correct notions of that which does exist. 



From this theory of IDEAS Plato derives natural theology, moral 

 philosophy, metaphysics, and logic, and we shall perhaps best exem- 

 plify the meaning of his theory by a few specimens of his reasoning. 

 " Nothing takes place without a cause. Now there are two kinds of 

 causes, mechanical or physical, and free or intelligent. The first are 

 subordinate and depend on other causes, which again must derive 

 their power of action from some absolute first cause, which depends 

 on no pre-existing condition, has nothing beyond it, cannot be pro- 

 duced and cannot disappear." This absolute first cause is the neces- 

 sary being whose existence is forced upon our attention, by consider- 

 ations independent of revealed religion, and whose power and 

 goodness and glory were therefore intelligible to every philosophic 



