Every medicine exhibits peculiar actions on the human frame, which are not produced in exactly the same manner by any other medicinal substance of a different kind.¹

¹ This fact was also perceived by the estimable A. v. Haller, who says (Preface to his Hist. stirp. helv.): "Latet immensa virium diversitas in iis ipsis plantis, quarum facies externas dudum novimus, animas quasi et quodcunque caelestius habent, nondum perspeximus."

As certainly as every species of plant differs in its external form, mode of life and growth, in its taste and smell from every other species and genus of plant, as certainly as every mineral and salt differs from all others, in its external as well as its internal physical and chemical properties (which alone should have sufficed to prevent any confounding of one with another), so certainly do they all differ and diverge among themselves in their pathogenetic - consequently also in their therapeutic - effects.¹ Each of these substances produces alterations in the health of human beings in a peculiar, different, yet determinate manner, so as to preclude the possibility of confounding one with another.²

¹ Anyone who has a thorough knowledge of, and can appreciate the remarkable difference of, effects on the health of man of every single substance from those of every other, will readily perceive that among them there can be, in a medical point of view, no equivalent remedies whatever, no surrogates. Only those who do not know the pure, positive effects of the different medicines can be so foolish as to try to persuade us that one can serve in the stead of the other, and can in the same disease prove just as serviceable as the other. Thus do ignorant children confound the most essential different things, because they scarcely know their external appearances, far less their real value, their true importance and their very dissimilar inherent properties.

² If this be pure truth, as it undoubtedly is, then no physician who would not be regarded as devoid of reason, and who would not act contrary to the dictates of his conscience, the sole arbiter of real worth, can employ in the treatment of diseases any medicinal substance but one with whose real significance he is thoroughly and perfectly conversant, i.e., whose positive action on the health of healthy individuals he has so accurately tested that he knows for certain that it is capable of producing a very similar morbid state, more similar than any other medicine with which he is perfectly acquainted, to that presented by the case of disease he intends to cure by means of it; for, as has been shown above, neither man, nor mighty Nature herself, can effect a perfect, rapid and permanent cure otherwise than with a homœopathic remedy. Henceforth no true physician can abstain from making such experiment, in order to obtain this most necessary and only knowledge of the medicines that are essential to cure, this knowledge which has hitherto been neglected by the physicians in all ages. In all former ages - posterity will scarcely believe it - physicians have hitherto contented themselves with blindly prescribing for diseases medicines whose value was unknown, and which had never been tested relative to their highly important, very various, pure dynamic action on the health of man; and, moreover, they mingled several of these unknown medicines that differed so vastly among each other in one formula, and left it to chance to determine what effects should thereby be produced on the patient. This is just as if a madman should force his way into the workshop of an artisan, seize upon handfuls of very different tools, with the uses of all of which he is quite unacquainted, in order, as he imagines, to work at the objects of art he sees around him. I need hardly remark that these would be destroyed, I may say utterly ruined, by his senseless operations.

Commentary:

Following points must be considered carefully

	Comparison,
	individualization, and
	difference in the nature of things most similar

The replacement of one remedy for another cannot be thought of, or entertained in Homoeopathy.

The homoeopathic physician must individualize, he must discriminate. He must individualize things widely different in one way, yet similar in other ways.

Lets take  remedy pulsatilla, sepia, Natrum muriaticum and Stannum metallicum and Ignatia in their mental state 

The Pulsatilla woman is tearful and easily discouraged. Sometimes she is full of anxiety, with forebodings of some imminent disaster. This anxiety comes from the epigastrium and is very likely to be linked with indigestion. It is often accompanied by chattering of the teeth, palpitation of the heart and flushes of heat. These are the main mental symptoms of Pulsatilla. They are present, more or less, in every disease in which it is the remedy. 
               

In these mental symptoms you should compare Sepia, which, you will recall, has a similar mental state; but it differs from Pulsatilla in the presence of irritability and anger. There is also indifference to her household affairs, to which she was formerly attentive.                                       

Natrum muriaticum also has tearful tendency similar to that of Pulsatilla, but consolation under Natrum mur. worsens, while under Pulsatilla the patient seeks consolation. 
Stannum metallicum likewise has this tearful disposition. The patient is very much discouraged, or is tearful over his chest symptoms. He fears that he will go into a decline. 
The Ignatia patient is sad, but she hides her grief from others. 

Now you can see that remedies are similiar in one way and different in another way. They are totally dissimilar as to the general state, whilst totally  similar as to particulars. Totality of pulsatilla is different from totality of sepia or totality of Natrum muriaticum or totality of Stannum metallicum or totality of Ignatia. When you take a general state they differ. personality of pulsatilla is not personality of sepia or personality of natrum muriaticum or personality of ignatia. THERE  ARE DIFFERENCES WHEN YOU CONSIDER MEDICINE AS A WHOLE. BUT WHEN YOU EXAMINE THE PARTICULARS ESPECIALLY COMMON SYMPTOMS THEY ARE ALL SAME. TEARFUL DISPOSITION IS FOUND IN ALL OF THE ABOVE MENTIONED REMEDIES.

Let me pen down some examples of kent ."Take for instance the two remedies, Secale and Arsenicum ; they are both chilly, but the patient wants all the covers off and wants the cold air in Secale, and he wants all things hot in Arsenicum. The two remedies thus separate at once ; they are wholly dissimilar as to the general state, whilst wholly similar as to particulars. A mere book-worm symptom hunter would see no difference between Secale and Arsenicum. You go to the bedside of a case of peritonitis, and you will find the abdomen distended, the patient restless ; you will find him often vomiting blood and passing blood from the anus ; you will find horrible burning with the distended abdomen, unquenchable thirst, dry, red tongue, lightning-like pulse. Well, Arsenicum and Secale have all these things equally ; they both have these things in high degree ; but when Secale is indicated he wants all the covers off, wants to be cold, wants cold applications, wants the windows open ; cannot tolerate the heat, and the warm room makes him worse. If Arsenicum is indicated in such a case, he wants to be wrapped up warmly, even in the month of july, wants hot food and hot drinks. The whole Materia Medica is full of these things and is based upon this kind of individualization. Without the generals of a case no man can practice Homceopathy, for without these no man can individualize and see distinctions."

After collecting all the particulars, one strong general (totality) rules out one remedy and rules in another. Physicians by the questions they ask often show that they have not been able to apprehend this idea of individualization. Kent has wittingly said "They pick out two symptoms, or one symptom common to two remedies, and say," Now, both of these remedies have this same symptom, how are you going to tell them apart ?" Well, if you are acquainted with the Materia Medica, with the art of individualization, you will at once easily see how to get the generals ; the generals of one are so and so, and the generals of the other are so and so, and this will enable you to distinguish one of these remedies as best adapted to the constitution, when the two remedies have the one symptom in any equal degree."

Now, this rules out the idea of replacement of one remedy for another. If one does not work, they say, try all down the list alphabetically, until you attain it.

Why a remedy that has never been known to bring forth that symptom may cure the case, because it is more similar to the generals of that case than any other. This is the art of utilising the Materia Medica. Many times a patient brings out that which is so strange and rare that it has never been found in any remedy.

You have to examine the complete case and see which remedy of all remedies is most similar to the patient himself. From beginning to end, the homoeopath must study the patient .

Read the aphorism 118 again : "Each medicine produces particular effects in the body of man, and no other medicinal substance can create any that are precisely similar."

There are cases that are so complex  that man, no matter how much he studies, cannot see the dissimilarity ; but, remember one thing, there is one remedy that is needed in the case, whether it is known or not ; it is needed in the case, and it has no substitute, for that remedy differs from all other medicines, just as this individual differs from all other individuals - Individualization.

That shows the necessity of waiting and watching. In Homoeopathy medicines can never replace each other, nor one be as good as another.

                                                         § 120

In this aphorism Hahnemann gives the reason why we should prove a drug. He says that everything of man i.e. life and  death, disease and  health depends upon medicine and hence medicines must be thoroughly  distinguished from one another and for this purpose each medicines must be  tested by careful, pure experiments on the healthy body for 

	the purpose of ascertaining their powers and real effects, in order to obtain an accurate knowledge of them, and
	to aid us to avoid any fault  in their utilisation in diseases, for it is only by correct selection of them that the greatest of all earthly blessings, the health of the body and of the mind, can be rapidly and permanently restored.

§ 121

In proving medicines to ascertain their effects on the healthy body, it must be borne in mind that the strong, heroic substances, as they are termed, are liable even in small doses to produce changes in the health even of robust persons. Those of milder power must be given for these experiments in more considerable quantities; in order to observe the action of the very weakest, however, the subjects of experiment should be persons free from disease, and who are delicate, irritable and sensitive.

Commentary:

Dose needed for proving a homoeopathic drugs varies according to drugs. Very powerful drugs (heroic drugs) even in small doses produce symptoms even in Physically strong persons . Those of milder drugs require larger quantity  of medicines  even in the weakest person. The conditions necessary for proving of drugs in both of the above cases are 

	persons free from disease
	delicate, irritable and sensitive.

The golden rules of drug proving :was laid by Herbert A Roberts in his THE PRINCIPLES AND ART OF CURE BY HOMOEOPATHY. He says From an apparently inert substance, such as Lycopodium, Carbo veg. or Graphites, we can obtain a good proving only from a high potency; therefore we may take as an axiom: ANY DRUG WHICH IN ITS NATURAL STATE WILL NOT AFFECTS THE VITAL ENERGY, WILL DEVELOP A PROVING ONLY IN A HIGH POTENCY. I have modified  Herbert A Roberts statement here. Other drugs having a very strong action upon the human economy in their natural state, such as Lobelia, Ipecac., Cicuta or Tabacum, may be used in a crude form: ANY DRUG WHICH IN ITS NATURAL STATE DISTURBS THE VITAL ENERGY TO FUNCTIONAL MANIFESTATIONS ONLY MAY BE PROVEN IN A CRUDE FORM Still other drugs, such as the Mercurius group, which are actively poisonous in the crude form, can be proven only in the high potencies. ANY DRUG WHICH IN ITS NATURAL STATE DISTURBS THE VITAL ENERGY TO DESTRUCTIVE MANIFESTATIONS SHOULD BE PROVEN ONLY IN A POTENTIATED FORM.