How rain is known as line and circle

One of the internal perceptive faculties that are proper to the living animal is fantasy [Gk. phantasia], which is the “common sense.” Its place is in the first chamber of the brain, where it receives into itself all that is conveyed to it from the totality of forms impressed on the five senses. Another faculty is positioned in the rear part of this chamber: this is the imagination [Ar. al-khayyāl], which is the form-producing faculty. It preserves what the common sense presents to it from the five individual senses, out of what remains there after the sensible objects are no longer present. It is well known that reception and preservation are the work of separate faculties. Think of water, which has the capacity to receive an imprint or an inscription and to take on any shape, but lacks the capacity to retain them (in a way that we will clarify later).

If you want to know the difference between the action of external sense generally, the action of the common sense, and the action of [the form-producing capacity of] the imagination, consider how you see the falling drop of rain as a straight line, and how you see the end-point of that cylindrical straight line as a circle. A thing cannot be perceived as a line or a circle until it has been looked into multiple times. External sense is unable to see it at two [separate] times, but sees it rather as it happens to be.
What is impressed on the common sense and passes away is perceived by the external sense as it happens to be. After it is effaced, its form remains in the common sense, which perceives it as if it were still there, just as when it happened to occur. The form-producing faculty sees an extended body which is spherical or straight, and this is something that can in no way be attributed to the external sense. The form-producing faculty perceives both matters, and gives form to them both, even when the thing itself no longer exists or is absent.

Then there is the faculty called "post-imaginative" with respect to the animal soul, and "cogitative" with respect to the human soul. This faculty is positioned in the central chamber of the brain, at the cerebellar vermis. Its characteristic function is that it combines and separates the contents of the imagination as it will. Then there is the estimative faculty, positioned at one end of the brain’s middle chamber, where it perceives the meanings [Ar. ma'ānī] which are not perceived through particular sensations. Such is the [above-mentioned] faculty present in the sheep, which determines that this wolf is to be fled from and that this lamb is to be cared for. It seems that this faculty is also what effects changes in imaginary objects, combining and dividing among them. Then there is the retentive, recollective faculty located in the hindmost chamber of the brain, which retains what the estimative faculty perceives in the way of meanings unperceived through particular sensations. The relationship of the retentive faculty to the estimative faculty is like the imagination’s relationship to the senses, and the relation of that faculty [the estimative] to the ma'ānī is like the relation of this faculty [the imaginative] to the sensible forms. These are the internal perceptive faculties of the animal soul.

On the soul (The Cure: Physics, book 6) I.5

Avicenna on extra-sensory perception

Reminder. The common sense is a tablet. When its surface is occupied by an impression, that impression is also made in the judgment of the beholder. Sometimes when the sensible object withdraws from the senses, its form remains for a while in the common sense and in the perceiver’s judgment, in isolation from the imagination. In this connection, you should recall what was said to you about the rain that falls in a straight line, and the circular impressions left by its scattered droplets. When, on the tablet of the common sense, the likeness of a form is produced, that form is then perceived, whether it is at the beginning of its state of being impressed from without, or persisting along with a persisting sensation, or as something fixed after that sensation has ceased. Alternately, [a form] might occur in the soul in a way that owes nothing to sensation.

Pointer. Forms that are present and available to external sense may be perceived by unwell and bilious sorts in isolation from the external sense-objects themselves, impressed on the soul through an internal cause which may or may not be influenced by some other cause. The common sense may also be stamped by a "wandering" form that strikes the seat of imagination and estimation; likewise, imagination and estimation may receive an impression from the tablet of the common sense. It is something like what happens between two mirrors that face each other.

Reminder. There are two causes that distract from this sort of impression. One is sensory and external; the other is produced internally by the intellect or the estimative faculty. In the former case, the tablet of the common sense is occupied by some other impression made upon it, effectively usurping [the attention] and wresting it away from the imaginary entity. In the latter case, the activity of the imagination is checked as the intellect or the estimation intervenes in what is occupying the soul, and the imagination is made compliant to it, and loses its power over the common sense. The imagination is no longer capable of making an impression on the common sense, for its movement is weak – something which follows, but is not itself followed.

If either of these two distracting faculties is stilled, the other one remains. It may happen that the remaining faculty is unable to check the imagination; in these cases, the imagination regains its power over the common sense, which is then taken over by signs of forms [such as are] beheld by the senses.

Pointers and Reminders 10.12-14

Avicenna on that helium

Reminder. From what has gone before, you know that the particulars are stamped on the intellectual world in their universal aspect. After that, you were led to notice that heavenly bodies are endowed with souls that have particular perceptions and desires proceeding from their particular point of view. There is no bar to their forming concepts of the particular concomitants that arise from their particular movements in the elemental world.

If this observation, concealed as it is from all but those whose intellect is stanched in elevated wisdom, is in fact true -- namely, that after the separated intelligences which are theirs as a matter of principle, they have a rational soul which takes in no material impression, and that furthermore these retain a certain connection, as between mortal souls and bodies, through which they are empowered to obtain a certain perfection; -- If this is true, then heavenly bodies are provided with a supplement, meaning that they present both a universal view and another view particular to them. What you should gather from these remarks is that the particulars do make an impression on the intellectual world in the form of a universal figure, and in the world of the soul the impression they make is time-sensitive and takes the form of a particular figure.

Pointers and Reminders 10.9