On the Improvement of Understanding, by Benedict de Spinoza, , at sacred-texts.com
 (1) Let us then make a beginning with the first part of the method, which is, as we have said, to distinguish and separate the true idea from other perceptions, and to keep the mind from confusing with true ideas those which are false, fictitious, and doubtful. (2) I intend to dwell on this point at length, partly to keep a distinction so necessary before the reader's mind, and also because there are some who doubt of true ideas, through not having attended to the distinction between a true perception and all others. (3) Such persons are like men who, while they are awake, doubt not that they are awake, but afterwards in a dream, as often happens, thinking that they are surely awake, and then finding that they were in error, become doubtful even of being awake. (4) This state of mind arises through neglect of the distinction between sleeping and waking.
 (1) Meanwhile, I give warning that I shall not here give essence of every perception, and explain it through its proximate cause. (2) Such work lies in the province of philosophy. (3) I shall confine myself to what concerns method—that is, to the character of fictitious, false and doubtful perceptions, and the means of freeing ourselves therefrom. (4) Let us then first inquire into the nature of a fictitious idea.
 (1) Every perception has for its object either a thing considered as existing, or solely the essence of a thing. (2) Now "fiction" is chiefly occupied with things considered as existing. (3) I will, therefore, consider these first—I mean cases where only the existence of an object is feigned, and the thing thus feigned is understood, or assumed to be understood. (4) For instance, I feign that Peter, whom I know to have gone home, is gone to see me, [r] or something of that kind. (5) With what is such an idea concerned? (6) It is concerned with things possible, and not with things necessary or impossible.
 (1) I call a thing impossible when its existence would imply a contradiction; necessary, when its non-existence would imply a contradiction; possible, when neither its existence nor its non-existence imply a contradiction, but when the necessity or impossibility of its nature depends on causes unknown to us, while we feign that it exists. (2) If the necessity or impossibility of its existence depending on external causes were known to us, we could not form any fictitious hypotheses about it;
 (1) Whence it follows that if there be a God, or omniscient Being, such an one cannot form fictitious hypotheses. (2) For, as regards ourselves, when I know that I exist, [s] I cannot hypothesize that I exist or do not exist, any more than I can hypothesize an elephant that can go through the eye of a needle; nor when I know the nature of God, can I hypothesize that He or does not exist. [t] (54:3) The same thing must be said of the Chimaera, whereof the nature implies a contradiction. (4) From these considerations, it is plain, as I have already stated, that fiction cannot be concerned with eternal truths. [u]
 (1) But before proceeding further, I must remark, in passing, that the difference between the essence of one thing and the essence of another thing is the same as that which exists between the reality or existence of one thing and the reality or existence of another; therefore, if we wished to conceive the existence, for example, of Adam, simply by means of existence in general, it would be the same as if, in order to conceive his existence, we went back to the nature of being, so as to define Adam as a being. (2) Thus, the more existence is conceived generally, the more is it conceived confusedly and the more easily can it be ascribed to a given object. (55:3) Contrariwise, the more it is conceived particularly, the more is it understood clearly, and the less liable is it to be ascribed, through negligence of Nature's order, to anything save its proper object. (4) This is worthy of remark.
 (1) We now proceed to consider those cases which are commonly called fictions, though we clearly understood that the thing is not as we imagine it. (2) For instance, I know that the earth is round, but nothing prevents my telling people that it is a hemisphere, and that it is like a half apple carved in relief on a dish; or, that the sun moves round the earth, and so on. (56:3) However, examination will show us that there is nothing here inconsistent with what has been said, provided we first admit that we may have made mistakes, and be now conscious of them; and, further, that we can hypothesize, or at least suppose, that others are under the same mistake as ourselves, or can, like us, fall under it. (4) We can, I repeat, thus hypothesize so long as we see no impossibility. (56:5) Thus, when I tell anyone that the earth is not round, &c., I merely recall the error which I perhaps made myself, or which I might have fallen into, and afterwards I hypothesize that the person to whom I tell it, is still, or may still fall under the same mistake. (6) This I say, I can feign so long as I do not perceive any impossibility or necessity; if I truly understood either one or the other I should not be able to feign, and I should be reduced to saying that I had made the attempt.
 (1) It remains for us to consider hypotheses made in problems, which sometimes involve impossibilities. (2) For instance, when we say—let us assume that this burning candle is not burning, or, let us assume that it burns in some imaginary space, or where there are no physical objects. (3) Such assumptions are freely made, though the last is clearly seen to be impossible. (4) But, though this be so, there is no fiction in the case. (57:5) For, in the first case, I have merely recalled to memory, [x] another candle not burning, or conceived the candle before me as without a flame, and then I understand as applying to the latter, leaving its flame out of the question, all that I think of the former. (6) In the second case, I have merely to abstract my thoughts from the objects surrounding the candle, for the mind to devote itself to the contemplation of the candle singly looked at in itself only; I can then draw the conclusion that the candle contains in itself no causes for its own destruction, so that if there were no physical objects the candle, and even the flame, would remain unchangeable, and so on. (7) Thus there is here no fiction, but, [y] true and bare assertions.
 (1) Let us now pass on to the fictions concerned with essences only, or with some reality or existence simultaneously. (2) Of these we must specially observe that in proportion as the mind's understanding is smaller, and its experience multiplex, so will its power of coining fictions be larger, whereas as its understanding increases, its capacity for entertaining fictitious ideas becomes less. (58:3) For instance, in the same way as we are unable, while we are thinking, to feign that we are thinking or not thinking, so, also, when we know the nature of body we cannot imagine an infinite fly; or, when we know the nature of the soul, [z] we cannot imagine it as square, though anything may be expressed verbally. (4) But, as we said above, the less men know of nature the more easily can they coin fictitious ideas, such as trees speaking, men instantly changed into stones, or into fountains, ghosts appearing in mirrors, something issuing from nothing, even gods changed into beasts and men and infinite other absurdities of the same kind.
 (1) Some persons think, perhaps, that fiction is limited by fiction, and not by understanding; in other words, after I have formed some fictitious idea, and have affirmed of my own free will that it exists under a certain form in nature, I am thereby precluded from thinking of it under any other form. (2) For instance, when I have feigned (to repeat their argument) that the nature of body is of a certain kind, and have of my own free will desired to convince myself that it actually exists under this form, I am no longer able to hypothesize that a fly, for example, is infinite; so, when I have hypothesized the essence of the soul, I am not able to think of it as square, &c.
 (1) But these arguments demand further inquiry. (2) First, their upholders must either grant or deny that we can understand anything. If they grant it, then necessarily the same must be said of understanding, as is said of fiction. (3) If they deny it, let us, who know that we do know something, see what they mean. (4) They assert that the soul can be conscious of, and perceive in a variety of ways, not itself nor things which exist, but only things which are neither in itself nor anywhere else, in other words, that the soul can, by its unaided power, create sensations or ideas unconnected with things. (5) In fact, they regard the soul as a sort of god. (60:6) Further, they assert that we or our soul have such freedom that we can constrain ourselves, or our soul, or even our soul's freedom. (7) For, after it has formed a fictitious idea, and has given its assent thereto, it cannot think or feign it in any other manner, but is constrained by the first fictitious idea to keep all its other thoughts in harmony therewith. (8) Our opponents are thus driven to admit, in support of their fiction, the absurdities which I have just enumerated; and which are not worthy of rational refutation.
 (1) While leaving such persons in their error, we will take care to derive from our argument with them a truth serviceable for our purpose, namely, [61a] that the mind, in paying attention to a thing hypothetical or false, so as to meditate upon it and understand it, and derive the proper conclusions in due order therefrom, will readily discover its falsity; and if the thing hypothetical be in its nature true, and the mind pays attention to it, so as to understand it, and deduce the truths which are derivable from it, the mind will proceed with an uninterrupted series of apt conclusions; in the same way as it would at once discover (as we showed just now) the absurdity of a false hypothesis, and of the conclusions drawn from it.
 (1) We need, therefore, be in no fear of forming hypotheses, so long as we have a clear and distinct perception of what is involved. (2) For, if we were to assert, haply, that men are suddenly turned into beasts, the statement would be extremely general, so general that there would be no conception, that is, no idea or connection of subject and predicate, in our mind. (3) If there were such a conception we should at the same time be aware of the means and the causes whereby the event took place. (4) Moreover, we pay no attention to the nature of the subject and the predicate.
 (1) Now, if the first idea be not fictitious, and if all the other ideas be deduced therefrom, our hurry to form fictitious ideas will gradually subside. (2) Further, as a fictitious idea cannot be clear and distinct, but is necessarily confused, and as all confusion arises from the fact that the mind has only partial knowledge of a thing either simple or complex, and does not distinguish between the known and the unknown, and, again, that it directs its attention promiscuously to all parts of an object at once without making distinctions, it follows, first, that if the idea be of something very simple, it must necessarily be clear and distinct. (3) For a very simple object cannot be known in part, it must either be known altogether or not at all.