Being, by which I mean, all of that which, when it is clearly and distinctly conceived is found to exist necessarily, or at least to be able to exist.
MT101-P03. PREV - NEXT - THIS - UPPER - TOPFrom this definition, or, if you prefer, from this description, it follows that chimeras, fictitious being and being of the reason can in no way be called real. For chimeras  by their nature do not exist. Fictitious being precludes any clear and distinct concept, because man by his mere power of Freedom, not unknowingly as in false concepts, but advisedly and intelligently, connects what he wishes to connect, and dissociates what he will. Finally, being of reason is nothing except a mode of thought which pertains most properly to the intellect [understanding], viz., to retention, to understanding [explanation], and to the imagination. It should here be noted that by mode of thought we mean, as was explained in Schol. Prop. 15 Pt. 1., all forms of mental states as understanding, joy, imagination, etc.
Chimeras, fictitious being, and being of reason are not real.
[Note N1]: By chimera is understood a being which by nature involves a contradiction as is clearly shown in Chapter 3.
MT101-P07. PREV - NEXT - THIS - UPPER - TOPIt is thus evident that such modes of thought are not ideas of things, nor can they, in any possible way be so considered. They have no object, which necessarily exists, as the source of the idea, nor could such an object possibly exist. The reason such forms of thought are so often held for ideas of things is that they arise so directly from real things, that those who do not very carefully attend to their thought readily confuse such forms of thought with the things themselves. For this cause also, they give names to these ideas as if they signified some real extra-mental object, which being, or rather non-being, they call beings of the reason.
Why beings of reason are not ideas of things but are so considered.
MT101-P08. PREV - NEXT - THIS - UPPER - TOPIt is easy to see how inapt is the division which divides Being into real being and being of the reason. For they divide Being into being and non-being or into being and a mode of thought. However, I do not wonder that philosophers sometimes fall into these verbal or grammatical errors. For they judge objects from the names and not names from the objects.
It is not correct to divide Being into real being and being of reason.
MT101-P09. PREV - NEXT - THIS - UPPER - TOPThose who say that being of the reason is nothing, however, are not less in error. If you seek for some meaning for these terms apart from the mind you find nothing; but if we understand by the term a mode of thought, then it signifies something real. For if I ask what a species is, I only inquire for the nature of that form of thought as something real and to be distinguished from other modes. These modes of thought, moreover, cannot be called ideas, nor can they be said to be true or false, just as love, e.g. cannot be called true or false but only good or evil. So when Plato said that "man is a biped without feathers," he did not err more than if he had said that man is a rational animal. For Plato knew that man was a rational animal as well as he knew the other. He merely put man into a certain class, so that when he wished to reflect upon man by referring to the class in which he had been classified he would come immediately to recognize certain characteristics as belonging to his nature. Aristotle, indeed, made a grave mistake if he thought that Plato in this definition attempted to express the essence of human nature. Whether Plato did well we may question, but this is not the place to discuss that.
In what sense being of the reason may be called nothing and in what sense real.
MT101-P10. PREV - NEXT - THIS - UPPER - TOPFrom all that has been said above it appears that there is no conformity between real being and being of reason. Therefore, it is easily seen how sedulously we must be on our guard lest we confuse the two. For it is one thing to inquire into the nature of things and quite another to inquire into the nature of the modes of thought under which they are perceived. If we do not keep this distinction clear we will be unable to understand modes of perception, or the nature of things in themselves. But what is more important, since this affects so many things, is that this is the reason we often fall into such great error.
In our investigation of things, real being must not be confused with being of reason.
MT101-P11. PREV - NEXT - THIS - UPPER - TOPIt should be noted also that many confuse being of reason and fictitious being. They think that the one is equal to the other because neither has an extra-mental existence. But if they would consider the definitions of each, great and important differences would be found, not only in respect to their cause, but in their nature apart from their cause.
In what way being of reason and fictitious being are distinguishable.
For we affirm that fictitious being is nothing but two terms connected
by the mere act of volition without any
dependence upon reason.
Being of reason
does not depend upon the
will alone nor is it formed by terms, as is
evident without a rational connection between them, from the
definition itself. If one should ask, therefore, whether
fictitious being, or being of reason is real it should be answered
that it is wrong to divide all being into
real being and
being of reason. The question is fundamentally wrong for it
presupposes that all being is divided into
real being and
being of reason.
Further, in order that we may the better understand what has been said and what is to follow, we will attempt to explain briefly what is meant by the terms essence, existence, idea, and power. We are the more urged to do this by the ignorance of those who do not recognize the distinction between essence and existence, or if they do recognize it still confuse the terms essence with the terms idea or power. Therefore, in order to help them and to make the matter plain we attempt to explain this as clearly as possible.