The affective part of the soul, therefore, is not corporeal; it is a form indeed, but a form engaged in matter, such as the appetite, the power of growth, both nutritive and generative, a power which is the root and principle of appetite, and the affective part of the soul. Now a form cannot undergo an affection or a passion, but must remain what it is. It is the matter (of a body) which is capable of being affected by a "passion" (an affection), when this affection is produced by the presence of the power which is its principle. Indeed it is neither the power of growth that grows, nor the nutritive power that is fed; in general, the principle that produces a motion is not itself moved by the movement it produces; in case it were moved in any way, its movement and action would be of an entirely different nature.[46] Now the nature of a form is an actualization, by its mere presence producing (something), just as if the harmony alone could cause the vibration of the strings of a lyre. Thus the affective part (of the soul, without itself being affected) is the cause of the affections, whether the movement proceed from it, that is, from sense-imagination, or whether they occur without (distinct) imagination. THE AFFECTIONS OF THE SOUL COMPARED TO A MUSICIAN PLAYING THE LYRE. We might further consider whether, inasmuch as opinion originates in a higher principle (of the soul), this principle does not remain immovable because it is the form of harmony, while the cause of the movement plays the role of the musician, and the parts caused to vibrate by the affection, that of the strings; for it is not the harmony, but the string that experiences the affection; and even if the musician desired it, the string would not vibrate unless it were prescribed by the harmony. PASSIONS ARE PRODUCED BY EXTERNAL IMAGES; AND THEIR AVOIDANCE IS THE TASK OF PHILOSOPHY. 5. If then, from the very start, the soul undergo no affections, what then is the use of trying to render her impassible by means of philosophy? The reason is that when an image is produced in the soul by the affective part, there results in the body an affection and a movement; and to this agitation is related the image of the evil which is foreseen by opinion. It is this affection that reason commands us to annihilate, and whose occurrence even we are to forestall, because when this affection occurs, the soul is sick, and healthy when it does no occur. In the latter case, none of these images, which are the causes of affections, form within the soul. That is why, to free oneself from the images that obsess one during dreams, the soul that occupies herself therewith is to be wakened.[47] Again, that is why we can say that affections are produced by representations of exterior entities, considering these representations as affections of the soul. PSYCHOLOGICAL PROCESS INVOLVED IN PURIFYING THE SOUL, AND SEPARATING SOUL FROM BODY.