When not long ago, while [Henry I of England (1068-1135)] was on the throne [1100-1135], I returned to
England after my long period of study abroad, it was very agreeable to
meet my friends again.... [A] nephew of mine who was with the others--he
was interested rather than expert in natural science--urged me to disclose
something new from my Arab studies. To this, when the rest had agreed,
I delivered myself in the tract that follows...
||...if I were only to listen to you expounding a lot of Saracen theories,
and many of them seemed to me to be foolish enough, I would get a little
restless ... [so] while you are explaining them, I will oppose you wherever
it seems fit. I am sure you praise them shamelessly and are too keen
to point out our ignorance. So for you it will be the fruit of your
labor if you acquit yourself well, while for me, if I oppose you plausibly,
it will mean that I have kept my promise.
WHY do plants spring from the earth?
If you collect dry dust and put it finely sieved in an earthenware
or bronze pot, after a while when you see plants springing up, to what
else do you attribute this but to the marvelous effect of the wonderful
||I do not detract from God. Everything that is, is from him and
because of him. But [nature] is not confused and without system and
so far as human knowledge has progressed it should be given a hearing.
Only when it fails utterly should there be recourse to God....
Those who are now called authorities reached that position first by
exercise of their reason.... Wherefore, if you want to hear anything
more from me, give and take reason....
WHAT theory is to be held concerning
Many different views have been expressed about the nature of sight,
and it will be perhaps convenient first to set them forth, and then inquire
which of them is the most reasonable....
||The theories I have been able to collect in various quarters about
sight fall into four different groups.
Some say that the mind, sitting in the brain is its chief seat, and looking
forth upon outer things through open windows, viz., the eyes, gets knowledge
of the shapes of things, and when it has got knowledge of them, judges
them; it being always understood that nothing from the mind passes to the
outside, and nothing from the shapes outside makes its way to the mind.
Others, again, maintain that sight takes place through the approach of
shapes, saying that the shapes of things give shape to the air that intervenes
between themselves and the eyes, and that in this way the materials for
judgment pass to the mind.
Very many also assert that something is sent forth by the mind, i.e., visible
breath, and that the shapes of the things that are to be seen meet it in
mid-air: having taken shape from these, the breath returns to its
seat, and presents the shape to the mind for it to exercise judgment upon.
A fourth party maintains that no shapes of objects approach the eye, but
that something which they call "fiery force," and which is produced in
the brain by means of concave sinews, passes first through the eyes, and
then to the objects to be seen, and by returning to its point of origin
brings back to the mind, with the same quickness as it went, the shape
impressed upon it as though by a potter....
||[T]here is another consideration which, even if we put other objections
on one side, is sufficient to upset all the views we have mentioned.
It is this: we are familiar enough with the sight of our own shapes
in a mirror; but this, though its reality is established by everyday life,
does not agree with the theories we have recounted. It will be better,
therefore, to deal with the view of which philosophy approves, and dismissing
other theories as lacking strength, put our faith in this academic truth.
This theory is as follows: In the brain there is generated a certain
air of the most subtle nature, and made of fire, and consequently exceedingly
light: this makes its way from the mind along the nerves, whenever
it so pleases, and necessity arises, to see things outside. Hence
it is called by physicists "visible spirit": being a body, it naturally
requires a local exit, which exit it finds through the different concave
nerves, which the Greeks call "optic," extending from the brain to the
eyes: then traveling to the body to be seen, it makes its way with
wondrous speed, and being impressed with the shape of the body, it both
receives and retains the impression, and then returning to its original
position, it communicates the shape it has received. Now this spirit
is called by philosophers "fiery force," and this force, when it finds
a mirror opposite to it, or any other light-giving body, being reflected
by it, returns as a result of the reflection to its own face, and still
retaining the shape, when it enters, reveals it to the mind. You
are not, however, to suppose that this fiery force found the shape of the
face in the mirror; but, being reflected from the surface which is too
smooth for it to abide there, received the shape while returning, and having
received it, brought it back.
This then is the divine theory which Plato has adopted, among other
things, in his Timaeus. "There are, in my opinion," says Plato,
"two virtues in fire, one consuming and destructive, the other soothing
and endowed with harmless light. With this one, therefore, in virtue
of which light bringing in the day unfolds itself, the divine powers are
in harmony, for it has been their pleasure that the intimate fire of our
bodies, own brother of the fire which is a passing bright, clear, and purged
fluid, should flow through the eyes, and issue from them in order that
through the eyes, slight, cramped, and affrighted, as it were, by the stouter
substance, but yet offering a narrow medium, the more subtle clear fire
might flow down through the same medium. Hence, when the light of
day lends itself to the diffusion of sight, then no doubt the two like
lights meeting in turn cohere into the appearance of a single body, in
which the flashing brightness of the eyes meet, while the intimate brightness
of the diffusion as it spreads is reflected by meeting with the image at
close quarters. All this then goes through one and the same experience,
and the result of that same experience, when either it touches something
else, or is touched by it; moved by this contact, it spreads itself through
the whole body, and making its way through that body to the mind and produces
the sense which is called 'sight.'"
WHETHER visible breath is a substance
or an accident?
I have now a task to impose both on Plato and on you, and I think it
will annoy you both: for if, as has been previously explained, anything
issues from the brain, whether with the physicists we call it visible breath,
or--as Plato insists--the fire of our own bodies, it must necessarily be
either substance or accident.
||It is corporeal substance; for, as the philosopher says, fire is a
most subtle substance, composed of the four elements.
HOW can this same breath in so short
a time travel to the stars and return from them?
[T]his fiery breath with a single glance of the eye, so to speak, sees
the stars; and we have to admit, that a body in so short a time traverses
and returns through as great a density of air as there is between us and
the moon and the infinite space that lies between us and the moon, and
between the moon and the sun up to the very aplanos itself: and this
is mere madness and quite impossible, since the actual breadth of the whole
earth bears no sort of proportion to the infinite diameter of the sky.
||That is the sort of difficulty that a man gets into who knows nothing
of nature: do you then pay careful attention and store up in your
mind what I am going to say. Just as of spaces some are wide, some
wider, and some widest of all, so of bodies some I hold are swift, others
swifter, and others swiftest of all. This, however, not everyone
can understand. Just as the extent of the sky and its shape as laid
down by geometers is barely or not at all clear to the vulgar mind, so
to it such a great speed in so short a time cannot be clear, though the
eye in such people is swifter than the mind; for they measure, or rather
mismeasure, everything according to the fallacious evidence of their senses
in terrestrial matters. They think that the size of the sky exactly
coincides with the earth, and that the bulk of the moon and sun and other
things, which true reason would show them to exceed in size the earth,
are not one whit greater than they seem to their bleary eyes.
Those, however, who in matters of this sort are more apt to use reason,
the incorporeal eye of the mind as their guide, ... see clearly the revolution
of the heavens and the unutterably swift movement of the visible breath--and
this in both cases, thanks to the use of reason.... [T]his visible
breath is more subtly perfected by the wonderful energy of creative virtue
than all things compounded of elements. Just as the traveling forth
of mind is the cause of that swift revolution, so the traveling forth of
body is the reason of this swift going and returning, and it is not strange
that the man who is ignorant of this should also be ignorant of and wonder
at its effects....
HOW is it that while the eye is
shut, the visible breath is not left outside?
Just imagine that while the visible breath is touching a star, the
eye is shut. This is bound to happen, frequently, and the breath
will then be left outside.
||If only you would remember what I said previously, you
would not raise this objection. The breath is very swift, and, as
said above, it is established that it is sent by the mind, and through
the mind. It is clear that the eye can be closed only by the mind,
by which all voluntary motion is communicated to the body; let the consequence
then of this also be clear, that that which sends also receives; and when
it wills, shuts the door in such a way as to involve no injury to itself;
and so that nothing of its own may remain outside, especially too when
the thing itself is of such speed that it immediately receives it back
WHYthe breath does not hinder itself
in going and returning.
Assume that this breath, or if you will, fire, makes its way as far
as to what is to be seen by it: is sight effected before it returns
||It brings no message until its return.
||If therefore sight takes place immediately after its return, when we
look at anything for a long while with fixed gaze and see it, it follows
that it has both gone and returned at the same time, if we see the thing
continuously and without any interval: hence both its going and returning
will be simultaneous and without division, and therefore it hinders itself
while going and returning.
||Nay, it is you who hinder yourself, for as a result of your not understanding
you make foolish objections.... When we say that it goes and returns
so quickly, that when one return is accomplished, another starting takes
place without a word, because there is no delay perceptible to any sense,
since there is nothing that is manifest to sense, i.e., to the intellect,
there is therefore no great need that it should go with the same speed
after its return as it returned with after its going. As, however,
this interval is not discernible to sense, the journey wrongly seems continuous.