Part II – Essay on a New Principle (1796)
The ?rst published result of Hahnemann’s new observations on medicine was a lengthy and important work entitled, Essay on a New Principle for Ascertaining the Curative Powers of Drugs, published in 1796. Here we ?nd the initial insights that emerged from all the hard labors of the previous eight years, in particular the early discoveries of the dual nature of disease and medicinal action. 
Hahnemann begins, as we have in the previous chapter, by discussing the contributions of chemistry to medicine. He stresses that chemistry may help ?nd the medicinal powers of substances, but it cannot tell anything about its functions in the human body, which is of a living nature. For example, he stresses that the mixing of a drug with drawn blood in a test tube cannot tell us what will happen with that drug in the body itself, for
…the drug must… first undergo an infinity of changes in the digestive canal, before it can get (and that only by a most circuitous method) into the blood. (Lesser Writings, p.253)
He also points out the dif?culty of ascertaining the value of drugs for people in terms of their action on animals, as what can kill a person (such as a large dose of Nux vomica) will not harm a pig, for example.
Hahnemann cautions against trying to ?nd the medicinal value of drugs in their external appearance, this approach being, “as deceptive as the physiognomy is in indicating the thoughts of the heart.”
Two Kinds of Sources for Materia Medica – Clinical and Provings
Here we see a clear recognition of the “high value” of clinical discoveries, but also a recommendation for the testing of substances on healthy human beings as being the only methodical way of more certainly discovering the therapeutic value of substances yet untested or unknown.
Nothing remains for us but experiment on the human body. But what kind of experiment? Accidental or methodical?
The humiliating confession must be made, that most of the virtues of medicinal bodies were discovered by accidental, empirical experience, by chance; often first observed by non-medical persons. Bold, often over-bold, physicians, then gradually made trial of them.
I have no intention of denying the high value of this mode of discovering medicinal powers – it speaks for itself….
[However] Such a precarious construction of the most important science… could never be the will of the wise and most bountiful Preserver of mankind. How humiliating for proud humanity, did his very preservation depend on chance alone. No! it is exhilarating to believe that for each particular disease, for each morbid variety, there are peculiar directly-acting remedies, and that there is also a way in which these may be methodically discovered.
When I talk of the methodical discovery of the medicinal powers still required by us, I do not refer to those empirical trials usually made in hospitals, where in a difficult, often not accurately noted case, in which those already known do no good, recourse is had to some drug, hitherto either untried altogether, or untried in this particular affection, which drug is fixed upon either from caprice or blind fancy, or from some obscure notion, for which the experimenter can give no plausible reason, either to himself or others. Such empirical chance trials are, to call them by their mildest appellation, but foolish risks, if not something worse. (Lesser Writings, p. 258-259)
Hahnemann then summarizes the two sources of drug information in terms of their curative powers – provings and clinical trials.
The true physician, whose sole aim is to perfect his art, can avail himself of no other information respecting medicines, than
First – What is the pure action of each by itself on the human body?
Second – What do observations of its action in this or that simple or complex disease teach us? (Lesser Writings, p.264)
As for the second, Hahnemann states:
Would to God such relations [between a specific drug and a specific disease] were more numerous! (Lesser Writings, p. 264)
There is also a problem of having a standard
…whereby we may be enabled to judge of the value and degree of truth of their observations.
[And this standard] can only be derived from the effects that a given medicinal substance has, by itself in this and that dose developed in the healthy human body. (Lesser Writings, p. 264)
He refers to accidental poisonings or even deliberate trials that have been undertaken, often on criminals, which reports would form the foundation stone of a new materia medica.
Hahnemann realizes that the problem he faces is one of a qualitative nature (to discover the value of the medicinal substances already in use), rather than simply one of finding more medicinal agents. It was the nature of his genius that he was able to look beyond the more super?cial to the deeper issues.
As we already possess a large number of medicines… but concerning which we do not rightly know what diseases they are capable of curing… it may not at first sight appear very necessary to increase the number of our medicinal agents. Very probably all (or nearly all) the aid we seek lies in those we already possess (Lesser Writings, p. 259)
This issue of quantity (number of medicines) versus quality (theory and principle) will arise each time a signi?cant problem emerges in terms of cure for his system, both in the context of his discovery of the chronic miasms and his use of dual remedies.
Two Types of Disease
Hahnemann goes on to explain two types of disease, a concept that he will continue to develop throughout his lifetime.
It is only the very great simplicity and constancy of [constant nature diseases such as] ague and syphilis that permitted remedies to be found for them, [that] are, however, probably specific in both diseases… Our great and intelligent observers of disease have seen the truth of this too well, to require that I should dwell longer on this subject.
Now, when I entirely deny that there are any absolute specifics for individual diseases, in their full extent, as they are described in ordinary works on pathology, I am, on the other hand, convinced that there are as many specifics as there are different states of individual diseases, i.e., that there are peculiar specifics for the pure disease [tonic], and others for its varieties [pathic], and for other abnormal states of the system.” (Lesser Writings, p. 260-261, bold and parentheses added).
We observe a few diseases that always arise from one and the same cause, e.g., the miasmatic maladies; hydrophobia, the venereal disease, the plague of the Levant, yellow fever, smallpox, cow-pox, the measles and some others, which bear upon them the distinctive mark of always remaining diseases of a peculiar character; and, because they arise from a contagious principle that always remains the same, they also always retain the same character and pursue the same course, excepting as regards some accidental circumstances, which however do not alter their essential character.
These few diseases, at all events those first mentioned (the miasmatic), we may therefore term specific, and when necessary bestow upon them distinctive appellations.
If a remedy has been discovered for one of these, it will always be able to cure it, for such a disease always remains essentially identical in its manifestations (the representatives of its internal nature) and in its cause.
All the other innumerable diseases exhibit such a difference in their phenomena that we may safely assert that they arise from a combination of several dissimilar causes (varying in number and differing in history and intensity).
Hence it happens that with the exception of those few diseases that are always the same [tonic], all others are dissimilar [pathic], and innumerable, and so different that each of them occurs scarcely more than once in the world, and each case of disease that presents itself must be regarded (and treated) as an individual malady that never before occurred in the same manner, and under the same circumstances as in the case before us, and will never again happen precisely in the same way! (Medicine of Experience, Lesser Writings, p. 440-442)
Dual Action of Medicines
I. Most medicines have more than one action; the first a direct action, which gradually changes into the second (which I call the indirect secondary action). The latter Is generally a state exactly opposite of the former. [these refer to the “Erstwirkung” (initial action) and “Gegenwirkung” (counter-action) to be found later in the aphoristic Organon, viz. §64.] (Lesser Writings, p. 266)
Two Principles of Treatment
Hahnemann now introduces the three ways of restoring health, which is really a two-fold division: mechanical (removal of external cause) and internal medicine (based on two principles):
1. Removal of the cause, to the extent it is apparent: e.g., the convulsions produced by tapeworm are removed by killing the animal.
This object is above all criticism [Hahnemann calls it the royal road], though the means employed were not always the ?ttest for attaining it. (Lesser Writings, p. 261)
2. By the use of the principle of opposites:
By the second way, the symptoms present were sought to be removed by medicines which produced an opposite condition; for example, constipation by purgatives; in?amed blood by venesection, cold and nitre; acidity in the stomach by alkalis; pains by opium.
Hahnemann states that this approach can give temporary relief in acute, self-limiting diseases and is only justi?ed where no other way is possible. But it is not to be tried in chronic disease as stronger and stronger doses are necessary to provide relief and it should be abandoned.
I beseech my colleagues to abandon this method (contraria contrariis) in chronic diseases, and in such acute diseases as take on a chronic character; it is the deceitful by-path in the dark forest that leads to the fatal swamp. (Lesser Writings, p. 262)
3. Treatment by the principle of similars
The better, more discerning, and conscientious physicians, have from time to time sought for remedies…which should not cloak the symptoms, but which should remove the disease radically, in a word for specific remedies; the most desirable, most praiseworthy undertaking that can be imagined…
But what guided them, what principle induced them to try such remedies? Alas! only a precedent from the empirical game of hazard from domestic practice, chance cases…
Nothing then remains but to test the medicines we wish to investigate on the human body itself. (Lesser Writings, p.263)
Hahnemann goes on to venture the principle behind the value of provings, in order to provide a rational basis for medicine.
First he advances the axiom regarding the action of medicinal substances on healthy persons:
Every powerful medicinal substance produces in the human body a kind of peculiar disease; the more powerful the medicine, the more peculiar, marked and violent the disease.(Lesser Writings, p. 265)
Next he states the axiom that disease is cured in nature on the basis of the law of similars:
We should imitate nature, which sometimes cures a chronic disease by superadding another, and employ in the (especially chronic) disease we wish to cure, that medicine which is able to produce another very similar artificial disease, and the former will be cured; similia similibus. (Lesser Writings, p. 265)
Thus, one needs to know only three things, which is later reprised in Aphorism 3 of the Organon:
…the diseases of the human frame accurately in their essential characteristics… the pure effects of drugs, that is, the essential characteristics of the specific artificial disease they usually excite…[then finally matching the two by] choosing a remedy for a given natural disease that is capable of producing a very similar artificial disease.(Lesser Writings, p. 265)
This is summarized by Hahnemann into a further maxim:
…that in order to discover the true remedial powers of a medicine for chronic diseases, we must look to the specific artificial disease it can develop in the human body, and employ it in a very similar morbid condition of the organism which it is wished to remove.
This then leads Hahnemann to the analogous maxim:
…that in order to cure radically certain chronic diseases, we must search for medicines that can excite a similar disease (the more similar the better) in the human body.(Lesser Writings, p. 267)
It is interesting to note that here Hahnemann is concerned essentially with natural diseases, and also with what he calls chronic natural diseases (this category includes all those endless varieties of diseases that don’t seem to resolve themselves). Elsewhere he clari?es this in a footnote (no. 1 on p. 265 of Lesser Writings) as he considers that self-limiting natural diseases at this point can be handled easily enough by removing the original cause (if possible) or by removing any obstacles to cure (see p. 261-262). His concern was for those diseases that are not self-limiting and for which no “rapidly-acting speci?c” exists (this would seem to mean in the context, a constant remedy determined from the constant nature of the self-limiting natural disease). Where the cause is unknown and there is no known speci?c based on the law of similars, Hahnemann here allows that a remedy based on opposites can be used. He admits, however, that it is purely palliative.
At this point we can see the early and important distinction Hahnemann makes between the constant speci?c remedies (mainly homogenic at this point in his discoveries), which are derived clinically, and those to be determined by the process of provings and then matching the proving (arti?cial disease) symptoms and the symptoms of the natural disease. This is an early form of the duality of disease that we witness coming to fullness in the dual remedy discoveries of Aegidi and Boenninghausen (followed by Hahnemann and later, Lutze).  
Hahnemann also gives us in this seminal work a ?rm foundation to the famous experiment with China (quinine) in crude dose that he undertook and commented on in his 1790 translation of the well-known English physician, William Cullen’s Materia Medica.
In my additions to Cullen’s Materia Medica, I have already observed that bark, given in large doses to sensitive, yet healthy individuals, produces a true attack of fever, very similar to the intermittent fever, and for this reason, probably, it overpowers and thus cures the latter. Now after mature experience, I add, not only probably, but quite certainly. (Lesser Writings, p. 267)
Two Actions of a Medicine: Initial Action and Counter-action
Hahnemann now comes to a lengthy and crucial explanation of the difference between the two actions of a medicine (which, we must remember is an arti?cial disease, such that the dual action of the medicine is mirrored in the dual action of disease, or vice versa). It is this difference, based on his close observations, that provides the solid and rational basis for his determination that the principle of similars cures and the principle of opposites only palliates or suppresses.
This axiom [similia similibus] has, I confess, so much the appearance of a barren, analytical, general formula [which it had been until Hahnemann!], that I must hasten to illustrate it synthetically. But first let me call to mind a few points.
Most medicines have more than one action; the first a direct action, which gradually changes in the second (which I call the indirect secondary action). The latter is generally a state exactly the opposite of the former.
Opium may serve as an example. A fearless elevation of spirit, a sensation of strength and high courage, an imaginative gaiety, are part of the direct primary action of a moderate dose on the system: but after the lapse of eight or twelve hours an opposite state sets in, the indirect secondary action; there ensue relaxation, dejection, diffidence, peevishness, loss of memory, discomfort, fear. (Lesser Writings, p. 266)
Thus, if one gives a substance that has a direct action opposite to the natural disease, this is followed by the indirect action, which is similar to the disease.
Palliative remedies do so much harm in chronic diseases, and render them more obstinate, probably because after their first antagonistic action they are followed by a secondary action, which is similar to the disease itself.(Lesser Writings, p. 267)
However, if one gives a substance
…whose direct primary action corresponds to the disease, the indirect secondary action is sometimes exactly the state of body sought to be brought about… (Lesser Writings, p.266)
Length of Initial and Counter-Actions of a Drug
Since we have now uncovered the secret of the dual action of a medicine and the need to match the initial action of both the medicine and the disease on the basis of principle (similars), it becomes important to know when the one action ends and the other begins. Hahnemann gives us various indications of the length of time of each action. We must remember here that he is using relatively large doses by today’s standards, but moderate by the standards of his day.