Law of similia is one of the cardinal principles of homoeopathy .
Logically arrived at, this healing principle is a true guide in the art of healing. Law of similia has its confirmation in the countless tests over the centuries and it has withstood the pressure of time and changing circumstances. Therefore it is rightly regarded as an universal principle .

DEFINITION
‘Similia similibus curenter’ is the Latin expression of the words ‘Let like be treated by likes’. Hahneman arrived at this therapeutic formula from the already existing law of nature ‘similia similibus curantar’ which means ‘like cures likes’. Hahnemann reached this law through the thread of inductive reasoning. He established the fact that this law is not of occasional occurance but of universal adaptability as a general law of permanent cure, for all physical and mental ailments of mankind.

DEVELOPMENT OF THE PRINCIPLE
This law was recognized from the ancient times. From time to time, all through the medical history the application of this law was demanded to a lesser or greater extend. Sometimes we find this law of nature to be decked out in some fantastic disguise and sometimes it is found to be buried in the mystic and obscure phraseology. The traces of the knowledge of this law could be made out in the writings of medical authorities of every age. Hints of its popular and domestic use could be made out from the medical records much before the time of Hahnemann and many has borne testimony to the truth of this principle.
No one knows exactly when this law was first recognized. Ancient Hindu manuscripts records its appliction. Hippocriates sensed the possibility of this law and applied it to many cases.

HIPPOCRATES
In one his treatise he admits that apart from the general rule of treatment contraria contraries the opposite rule also holds good in certain cases viz similia similibus curentar.
He illustrates that the same substance that causes strangury, cough, vomiting, and diarrhoea will cure these diseases. In his work De Morbis Popularies he says dolor dolom solvit, which means one pain cures other. In his de Morbo Sacro he says the same cures epilepsy, which produces it. In epistiles of Hippocrates he quotes , helleborus given to sane pours on the darkness of mind, but it is greatly to benefit the insane. Futher referance to the homoeopathic principle could be found in the book de internis affectionibus and in his aphorisms.

EMPIRICAL SCHOOL-
Empirical school shows many points of resemblance to Hahnemanic doctrines . Many like Erasistratos , Heraclides, Mithridates initiated experiments to ascertain the power of drugs. And many like Nicander recognized the homoeopathic principle. Many like Xenocrates , Varo , Celsus etc have illustrated examples which even though belongs to the domain of isopathy , suffices to show the existence of a sort of instinctive notion that the remedy must act in the same sense as the morbific agent. These were the rough and rude expressions of similia similibus

GALEN
Galen , the father of allopathic physic and also the champion of the motto contrario contraries curanter gives a striking testimony even with reservations about the occasional truth of the opposite maxim . There is a doubtful acknowledgment of homoeopathic principle in his similia similibus deus adjungit and many similar writings .

BASIL VALENTINE
Another authority Basil Valentine says like must be cured by likes and not by contraries. Hence poisonous minerals can cure poisonings, which they can bring about.


PARACELSUS
Theophrastus von Hohenhim popularly know as Paracelsus has also elaborated the ideas of similarity into a dependable therapeutic axioms with compelling logic and systematization Homoeopathic principle is more clearly set in his treatise von der Astronomy. “Nature wills that in the combat, stratagem should be employed against stratagem and in medicine also the same rule prevails”.

In Arcludoxisis he says, ‘What makes jaundice also cure jaundice. In like manner, the medicine that shall cure paralysis must proceed from that which causes it’.

CROLL
A follower of Paracelsus gives further proof of the notion of a necessary analog between disease and remedy.

JOHANN ARNDT
- a theologian witnesses that physicians can cure opposites with opposites and sometimes likes with likes, poisons with poison

DOCTRINE OF SIGNATURE
A rude testimony of a barbarous age to the truth of what we know to be one true law in medicine. This represents that the outward and visible form of a drug as expressed by its taste , smell , color and appearance will give an indication to its therapeutic use . The plant mandrake resembles a small child so it can be given for sterility . Euphrasia resembles the eye so it can be given for eye affections .

HAHNEMANN
But it was Hahnemann who made the idea of similarity in medicine definite and pregnant for the first time by conceiving and utilizing it to built up a therapeutic system in an elegant way.

Discovery of Similia
Dr. Hahnemann born at Leipzig was a visionary who revolutionized the art of healing. After his graduation in 1779 from Erlangn University, he started his practice as a devout and conscietious physician. The absurdities and eccentricities of the existent system made him relinquish his practice and he occupied himself exclusively with chemistry and translatory works. The insufficiency of medical sciences and the therapeutic method of his days became more evident to him and he denounced it with undaunted energy.

In 1790, Hahnemann took to the task of translating Cullen’s M.M. and this turned to be the first milestone in the road of development, of the new method of treatment. In question of the medicinal effects of Peruvian bark Cullen defended the old opinion of efficacy of this remedy through its astringent property and tonic effect on the stomach. Hahnemann attacked this opinion vigorously in his notes (Vol II ,pg 108): “By combining the strongest bitters and the strongest astringents we can obtain a compound which in small doses, possesses much more of both these properties than the bark, and yet in all eternity no fever specific can be made from such a compound”. Thus Hahnemann on translating Cullen’s M.M. became indignant over the affect theoretical explanations of the antipyretic power of cinchona bark and therefore resolved to ascertain by the natural method of experience and thereby made the first experiment upon himself .He took an ounce of cinchona bark and was surprised to see that the symptoms evolved were similar to marshy fever .

Hahnemann here sensed a law that thought him to recognize that the effect a substance has on healthy organism is its curative power. Hahnemann then conducted experiments upon his family members and followers and thus confirmed his conclusions . Thus from the particulars he was able to reach a general conclusion using the inductive method of reasoning . Hahnemann’s enquiry as to under what circumstance the drug is curative lead to a new way of proving drugs upon the healthy and also to the conclusion that it is not the astringent property of china but its ability to produce similar symptoms that gave it the curative power. Hahnemann enunciated this principle in 1796 in his essay On the new principles for ascertaining the curative power of drugs and some examination of its previous principles .And this lead to the birth of a new system of medicine
viz homoeopathy .

EVIDENCE OF SIMILIA
Hahnemann has confirmed the law of similars not only with experiments but also with observations of other medical and non medical persons .
In his introduction to organon of medicine he points out the experience of various persons who have founded the treatment on the principle of similarity to be only efficacious mode.
1) For recent cases of frost bitten limbs , frozen sour crout is applied or friction of snow are used
2) The experienced cook holds his hands at some distance from fire
3) Manufactures of lackered ware , apply to parts scalded with hot varnish a substance that cause similar burning sensation such as oil of turpentine or strong heated spirit of wine.
4) The old experienced reaper after working in the heat of sun takes a small quantity of heating liquor or a mouth full of brandy where by his heat and fatigue are easily removed .

Examples of physicians of an earlier period suspecting similia principle as the excellent mode of treatment
Some physicians vaguely summarized that medicines cure analogous morbid states by the power they possess to cure analogous morbid symptoms
Boulduc guessed that purgative property of rhubarb was the cause of its power to allay diarrhoea
Detharding guessed that infusion of senna leaves relieved colic by its power to cause colic in healthy persons .
Bertholon confesses that in diseases electricity diminishes and removes pain very similar to which it produces
Thoury testifies that positive electricity produces quickening of pulse but when it is already morbidly accelerated it diminishes its frequency .
Von stroerk makes a suggestion that if stramonium disorders mind and produces mania in healthy persons ought not we try it in cases of insanity for restoring reason by producing a revolution of ideas
Stahl , a Danish army physician , treatment by means of oppositely acting remedies is wrong and is reverse of what ought to be done

In the foot note to introduction Hahnemann speaks of isopathy , a system based on aequlia aequilibus as an indifferent subsitute of homoeopathy which Hahnemann says is nothing but physical homoeopathy .Many examples they allude to are homoeopathic and are falsely explained as isopathic eg . scalded hand healed by application of warm water of some what lower temperature .Muscular contractions caused by chill cured by cold bath .
In Lectures on theory and practice of homoeopathy , Dr Dudgeon says about isopathy as a diversion from homoeopathy encouraged by Dr Lux , which has nothing but aberrations and eccentricities to be attached to it . They owe all their fame and fortune to homoeopathy. Even though it had many zealous supporters among homoeopathic ranks, it naturally died out after being a nine day wonder . Isopathy is nothing more than a one sided employment of similarly acting medicines. This system also verifies to some extend the validity of law of similars

CONTROVERSY REGARDING SIMILIA
There exisisted a controvery regarding the exoposition of the phrase similia similibus curenter . Dr Dudgeon and Dr Hughes strongly advocates that similia similibus curenter should be interpreted as let likes be treated by likes rather than let likes be cured by likes
Dr Hughes explicitly announces that similia similibus curenter is a rule of art rather than a law of science . He says that we are not justified in affirming that all morbid states are cured by similars . Therefore this practical conclusions has their qualifications as well as exceptions. However this has been contradicted by many others like Dr Wells and Dr Reinke who states similia to be a law of nature which is universal , exclusive and unchanging .
Dr Dudgeon strongly emphasizes that Hahnemann used the word curenter to give a mandatory turn to the phrase . As Hahnemann was a good Latin scholar he would not have employed it erronousely . Further he refers it to as the guide to the true way of healing in the 1st edition of Organon

Scientificity of similia
As the law of similia confirms to all requirements in the derivation of a law we can pronounce it as a scientific law . Any law is a scientific law when it satisfies the following requisites .
1) collection of data
2) interpretation
3) formation of hypothesis
4) verification as a thesis

Collection of data
Excat observation led Hahneman to see that law of similars was dimly perceived by various physicians before his time like Hippocrates , Nicander , Xenocrates , Celsus, Galen , Valentine , Paracelsus etc . He got further proof in the records of provings and accidental cures . Many contemporaries also testified to the authencity of this principle. Hahnemann experimented upon himself , his family members and his followers .

Interpretation
Through the accumulation of more data Hahnemann was able to reduce the possibility of error and correctly interpret the observed facts. On the basis of observed data and experimentation , he was able to interpret ,guided by the inductive method of logic that the law of similars is a general law derived from various individual facts.

Formation of hypothesis
A rational explanation that disease are cured by medicines that have the power to excite a similar affections was put forward . This lead to the formation of a hypothesis which needed further verification to become a thesis.

Verification
This general law was applied to each and every particular case of disease and the results obtained were studied . Thus Hahnemann through inductive and deductive logic laid the foundation of a true scientific system .By the application of this general law many diseases even new and previously unknown were successfully treated . Many observed and recorded facts of clinical trials over the centuries abounding the literature of homoeopathy also testifies to the validity of this law .

Thus scientificity of similia was constructed through inductive logic and clinically verified through deductive method of logic.
Law of similia ranks equal to the universal law of natural sciences . As is requisite for a general law of science , this law also expresses a relationship between two series of phenomenon .
In physics the phenomenon of sun is related to phenomenon of earth by law of attraction . In chemistry , properties of potassium is related to properties of sodium through law of chemical affinity and definite proportion . Similarly drug phenomenon is related to disease phenomenon through law of similars .
As the law of simila is a universal law it affords us the means of prevision. It not only establishes a relationship between proved drug and known disease phenomenon but also between unexplored medical wealth and undeveloped medical requirements of the sick ie new and unknown diseases can be successfully treated by collecting its totality of symptoms .

This law as it is not based on empty speculations or hypothesis and therefore is not overthrown by the progress of scientific knowledge .

Ref books :
1) Lectures on theory and practice of homoeopathy - Dudgeon
2) Principles and practice of homoeopathy - Richard Hughes
3) Organon of medicine - Samuel Hahnemann
4) Genius of homoeopathy – Stuart Close
5) Samuel Hahnemann , His life and works - R . Hale
6)The principles and the art of cure by homoeopathy - H. A. Roberts