LH III 1, 1, ff. 1–3. Previously published in a critical edition in Pasini, Corpo e funzioni cognitive in Leibniz, 1996.
I am of the opinion that no one has yet written the Medical Institutions that I desire; yet I believe that today a learned doctor could easily write them. They would be as useful for teaching, exercising, and developing the art as a torch brought into a dark place.
It is evident that the human body is a machine disposed by its author or inventor to certain functions. And thus to write medicine is nothing other than to prescribe to a mechanic a method by which he will be able to conserve the machine that has been entrusted to his care, so that it should continue to operate correctly, like the precepts that are typically given to the custodians of those hydraulic machines by means of which water is dispensed throughout an entire city.
In any Machine one must consider both its functions or ends, as well as its manner of operating, or by which means the author of the machine achieved its end. And therefore we should take care lest we imagine a machine that would by chance fulfill these same functions, but nevertheless not by the same means, since the precepts governing the conservation of this imaginary machine were different from the laws governing the true machine. Thus it is not surprising that certain new philosophers, with whose very ingenious thoughts about human beings we are familiar, have contributed little to the advancement of medicine, since they have sketched out their man more from the intellect than from experience.
The primary function of a human being is perception, but the secondary function (which exists for the sake of the first) is to procure perceptions. The advancement of human perfection consists in the same measure in the advancement of these functions. And anything that is helpful to perception or to the faculty of procuring perceptions is agreeable; anything that impedes them is disagreeable.
The organs of sense exist for the sake of perception; and the organs of motion exist for the sake of procuring perceptions, which is to say for action. Both should be conserved in operation, or in the constant capacity for operation, which is brought about now by the removal of impediments, now by the increase of facilitating conditions. And the greatest increase is nutrition itself, seeing that the same individual particles cannot be conserved, but continually vanish.
The manner by which sensation is brought about can therefore be described through a separate consideration than the manner by which Motion, or the procuring of sensation, is brought about. It will be supposed, of course, that the object and the organs have been disposed in such a way as to sense; one could give an account on which there were in a human being no faculty of procuring for himself any perceptions, just as whoever is sleeping deeply has perceptions, but does not procure them for himself. From here it could be explained in what way perceptions excite in us the desire to procure other perceptions. We must thus describe the manner in which we are able to dispose the organs of sense as well as the objects in such a way that a certain perception arises in us. And this entire process may be understood even if nutrition is not yet understood, since it does not depend on it. Thus we sometimes see in those who are burdened by atrophy, or who cannot hold down their food, that sense and motion nevertheless endure for a sufficient length of time. Finally, therefore, this method would bring it about that the organs of nutrition would be revealed to be organs that are necessary not to present functioning, but only to future functioning. As for the organs of generation, they can be described as organs of motion by which an agreeable perception is procured, for nature brings it about that action the action of generation in animals be conjoined with the greatest pleasure.
In truth, insofar as our medicine is more concerned with the functions of nutrition than with those of voluntary movement, and more with those of voluntary movement than of sensation, and insofar as it is able to imagine a machine capable of nutrition, but deprived of sensation and of animal motion, it is thus easier to explain how we nourish ourselves than how we perceive and act. Ultimately, with respect to the manner in which the aliments serve to generate the parts we require in order to exercise the functions of the sensitive soul, it will be preferable to inquire first into those parts that are in a certain respect held in common even with plants, than into those that are proper to animals.
This method is analytic, moreover, when we investigate the means or the organs of any given function and the mode of their operating, and thus arrive at an acquaintance with the body through its parts. Once this task is completed, we will come back to synthesis, and we will describe all of the parts coordinated into one, and the first motor of motion and the liquid and solid instruments of motion, and their connection, and ultimately the entire economy of the animal, particularly when we will have learned by means of analysis the organs of each function, that often the very same organ can be devoted to several functions, just as in machines the wisdom of their maker shines through most of all when many effects are brought about by limited means.
In order to spell out this synthesis, we must consider what the ancients already had the wisdom to observe, that all of the parts of our body can be distinguished into what contains, what is contained, and what is an agent of impetus, which is to say into veins, humors, and spirits, and consequently our body is a hydraulico-pneumatico-pyrobolic machine. Now humors are contained in the vessels, but a spirit that penetrates, agitates, and moves everything is also diffused by the vessels, which is like the rays of the sun or a flame or even like an ignited cannon, or like other explosives of the sort we observe in fermentations and reactions (for all of this goes back to the fact that undoubtedly the perturbed ether tends to reestablish itself).
That in our bodies there are spirits—that is, a rapidly moving, imperceptible matter—is demonstrated not only from the circulation of the blood, but also from the members whose motion provides enough force for us to be able to suddenly lift great weights. This cannot be attributed to the structure of the machine, for machines raising great weights by means of forces, raise them slowly, while animals do it quite swiftly, from which it follows that they have a great force in them. Now it is clear that the members do not have a visible principle of motion, and even that the extravascular liquors do not cease to pursue their motion even when they have been perceptibly changed, from which it follows that the motor in our bodies is insensible. That there is in contrast a certain continuous fluid diffused throughout the whole body, from which it can be understood, that all things that are in contact with any given part of our bodies can easily be sensed. Once this spirit is set into a great internal motion, moreover, it does not lose force, but nor does it bring it about or communicate it to another, unless heavier bodies are immersed in it, or resist it, whereupon it will have a great force upon them; and when it is restrained from every side, if perchance an outlet toward somewhere else becomes available, it expends all of its force toward that place; from which it is apparent in what way by a slight exertion we exercise a great force: for to bring this force about it is not necessary to bring this force about in us, but only to direct it. Moreover, the spirit requires an aliment, as a flame requires oil; it requires air, which is to say it must respire, and moreover that it must also eliminate what it dissipates, and neither is it of importance that the blood of all animals is warm to our touch, for indeed we are not able to detect all luminous things, nor all things that effervescence makes warm, by our senses. And we know that a phosphorescent body, when it is deprived of air, gradually ceases to give off light, until such time as it is able to respire again.
It is not at all doubtful, therefore, that our life consists in something resembling a flame, which is found not so much in the heart as in all of the parts. That we should be the less astonished, we must know that a flame of this sort or a motive spirit is found in all bodies, that the force that it exerts comes from fermentations and other reactions or conflicts, that this spirit, once set in the appropriate motion, maintains the fluidity of water, but that, when it is more weakly agitated by the ambient bodies, the water freezes. In the same way, in us the spirit also receives its agitation from the ambient ether, with which it communicates in all places by means of small openings. The same spirit also easily conserves motion, for, in view of its divisibility, the obstacle that is opposed to a part of the body does not obstruct the motion of all of the other parts, from which it arises that soon, when the obstacle has been suppressed, the motion is communicated to that very part that had been obstructed.
Whenever the motion of this spirituous matter is disturbed, there arises in the liquid a certain ebullition, whose species is either fermentation, when one body is involved, or reaction, when two opposing bodies are mixed. Now the number of ebullitions of this sort in our body must be great, arising either when diverse humors are mixed with one another or when new aliments or new air are mixed into the contents that are already there.
I would thus suppose that a reaction arises when bile is mixed into the blood, for if a few drops of bile—even cold bile—are mixed into the blood outside of the vessels that has already coagulated and is turning dark, it will become a bit fluid and reddish. For indeed the blood is like oil, and the bile like sulfur, as sulfurs are easily dissolved by the oils that correspond to them, if alkalis have already been added (in fact there is in bile, just as in blood, a volatile alkali or urinous salt). As it is plausible that new aliments react with the blood confected from aliments already taken, for the bodies that are opposed to one another while also having a certain affinity are the ones that react most intensely, especially because +-------+ and the matters saturated by the bile somewhat earlier in the jejunum penetrate exactly from there into the blood.
It is not necessary initially to investigate the origin of the blood; indeed, it suffices that both the blood and its motion be communicated from the mother, for example, that the respiration of the mother is also beneficial to the infant; the blood is moved in the heart, veins, and arteries; it is dispersed in the arteries from the heart to the extremities [membra]; in the veins it returns from the extremities to the heart; it is filtered in the liver, where the bile is deposited, carried from the gall bladder into the jejunum through its own duct, where it saturates the chyle coming from the stomach, and with it returns to the blood in the subclavial veins.
In the extremities the blood deposits the lymph, which in turn is moved along in its own vessels, which are called “lymphatic.” It comes with the blood in the arteries from the heart but does not return to the heart with the blood through the veins. A part of the lymph serves for nutrition, the rest moves through the lymphatic vessels. The lymphatic vessels pass through the glands, which are like nodes or shallow basins in which the filtered lymph is transformed and acquires a new nature. Among these glands, the pancreas is distinctive, as it is a gland that is agglomerated out of many simple (or clotted) glands; in the pancreas a subacidic lymph, which is called pancreatic juice, is produced; this lymph is more easily transformed by air than the lymph that is found in other parts. Saliva is also a particular sort of lymph, which undoubtedly contributes much to digestion in the stomach. The seminal vessels also belong to this category, the semen being composed partly from the lymph and partly from the blood. For the blood deposits both urine and blood in the kidneys, into which lymph is in turn mixed in the testes. It appears that the brain and the marrow are stagnant, condensed lymph. It is certain that bones are perforated by various vessels; it remains to be seen whether arteries or veins deposit something there, and whether the lymphatic vessels convey lymph there and bring it back from there.
As lymph is quite close to being a nutritive juice, it is clear that it responds to nature’s intention and that the blood, deposing its heterogeneous components and consuming its sulfur as if by a flame in the course of its continual trajectory through the heart, transforms little by little into lymph. For, given all the sulfur that it has lost in passing through the heart and in being propulsed through the arteries, it is lymph ridden of sulfur that is deposited, once it cools down, in the extremities. The blood however, still saturated with the sulfur, yet still mobile, is carried to the heart by the veins.
The blood carried with the lymph to the thin vessels dispersed throughout the flesh is dissipated by sweat, after which these vessels, once again cooled down, absorb new matter coming from the blood: they purge the blood of it. This is why sweating is almost a panacea and stands almost alone in its usefulness for illnesses. +-------+ Matter is carried away by the sweat or by imperceptible perspiration in order to make room for new deposits. I understand completely that attraction whose origin is a certain initial impetus. +-------+.
All medicines operate either after the manner of aliments or of poisons. The former operate by degrees and insensibly, the latter by a great and sudden force. This is why the latter are not to be tried unless we require immediate aid.
The method of treating illnesses is twofold. One method is analytic, in view of the symptoms; the other is synthetic, in view of the causes. It must be considered that all symptoms are simple illnesses, for functions are always damaged by them. But since, when one function is damaged, it damages several others along with it, it can often be concluded that there is a single cause of multiple symptoms. If the damage done to the function is not perceptible in itself, then it is not deemed to be a symptom. The method of treating by symptoms would be infinite if we wished to enumerate all of their combinations. There are certain signs of a good constitution, and of a bad one, which cannot be called symptoms, such as color, urine, what is pleasing and what is injurious. One must teach true analysis, that is, the art of studying signs, on the one hand, and on the other of inferring the illness from the signs. Synthesis should be taught after having presented a model of analysis, that is, a model of the general method of treatment, which is to pathological synthesis what algebra is to the elements of geometry.