by Samuel Hahnemann (1796) As sources for ascertaining the medicinal virtues of drugs were so soon exhausted, the systematizer of the materia medica bethought himself of others, which he deemed of a more certain character. 
He sought for them in the drugs themselves; he imagined he would find in them hints for his guidance.
He did not observe, however, that their sensible external signs are often very deceptive, as deceptive as the physiognomy is in indicating the thoughts of the heart.
Lurid-coloured plants are by no means always poisonous; and on the other hand, an agreeable colour of the flowers is far from being any proof of the harmlessness of the plant. The special qualities of drugs, which may be ascertained by the smell and the taste, will not allow us to form any trustworthy conclusions respecting untried substances.
I am far from denying utility to both these senses in corroborating the probable properties of drugs which have been ascertained in other ways, but I would counsel, on the other hand, great caution to those who would form their judgment from them alone.
If the bitter principle strengthens the stomach, why does squill weaken it? If bitter aromatic substances are heating, why does marsh rosemary diminish the vital temperature in such a marked manner? If those plants only are astringent that make ink with sulphate of iron, how is it that the highly astringent principle in quinces, medlars, andc., cannot furnish ink?
If the astringent taste gives evidence of a strengthening substance, why does sulphate of zinc excite vomiting? If the acids are antiseptic, why does arsenious acid produce such rapid putrefaction in the body of one poisoned by it?
Is the sweet taste of sugar of lead a sign of its nutritive properties? If the volatile oils, and everything that tastes fiery on the tongue, are heating for the blood, why are either, camphor, cajeput oil, oil of peppermint, and the volatile oil of bitter almonds and cherrylaurel, the very reverse?
If we are to expect a disagreeable odour in poisonous plants, how is it so inconsiderable in monkshood, deadly nightshade, and foxglove? Why so imperceptible in nux vomica and gamboge?
If we are to look for a disagreeable taste in poisonous plants, why is the most deadly juice of the root of jatropha manihot merely sweetish, and not the least acrid?
If the expressed fatty oils are often emollient, does it follow that they are all so, even the inflammatory oil expressed from the seeds of the jatropha curcas? Are substances which have little or no smell or taste destitute of medicinal powers?
How is that ipecacuan, tartar emetic, the poison of vipers, nitrogen, and lopez-root, are not so? Who would use bryony-root as an article of diet, on the ground that it contains much starch?
Perhaps, however, botanical affinity may allow us to infer a similarity of action?
This is far from being the case, as there are many examples of opposite, or at least very different powers, in one and the same family of plants, and that in most of them. We shall take as a basis the most perfect natural system, that of Murray.
In the family of the coniferae, the inner bark of the fir-tree (pinus sylvestris) gives to the inhabitants of northern regions a kind of bread, whereas the bark of the yew-tree (taxus baccifera) gives-death.
How come the feverfew (anthemis pyrethrum), with its burning root, the poisonous cooling lettuce lactuca virosa), the emetic groundsel (senecio vulgaris), the mild scorzonera, the innocuous cudweed (gnaphalium arenarium), the heroic arnica (a. montana), all together in the one family of the compositae?
Has the purging globularia alypum anything in common with the powerless statice, both being in the family of the aggregatae? Is there any similarity to be expected betwixt the action of the skirret root (sium sisarum) and that of the poisonous water-dropwort (aenanthe crocata), or of the water-hemlock (cicuta virosa), because they are in the same family of the umbelliferae?
Has the not harmless ivy (hedera helix), in the family hederaceae, any other resemblance to the vine (vitus vinifera), except in the outward growth? How comes the harmless butcher’s-broom (ruscus) in the same family of the sarmentaceae with the stupifying cocculus (menispermum cocculus), the heating aristolochia, and the asarum europaeum?
Do we expect any similarity of effect from the goose-grass (galium aparine) and the often deadly spigelia marylandica, because they both belong to the stallatae? What resemblance can we find betwixt the action of the melon (cucumis melo) and the elaterium (momordica elaterium), in the same family of the cucurbitaceae?
And again, in the family solanaceae, how comes the tasteless great mullein (verbascum thapsus), along with the burning Cayenne pepper (capsicum annuum); or tobacco, which has such a powerful spasm-exciting action on the primae viae, with nux vomica, which impedes the natural motions of the intestines?
Who would compare the unmedicinal perriwinkle (vinca pervinca) with the stupifying oleander (nerium oleander), in the family contortae? Does the watery moneywort (lysimachia nummularia) act similarly to the marsh trefoil (menyanthes trifoliata), or the powerless cow-slip (primula veris), to the drastic sowbread (cyclamen europaeum), in the family of the rutaceae?
From the strengthening effects of the bear-berry (arbutus uva ursi) on the urinary apparatus, can we infer the heating, stupifying action of the rhododendron chrysanthum, in the family bicornes?
Among the verticillatae, can any comparison be made betwixt the scarcely astringent self-heal (prunella vulgaris) or the innocent bugle (ajuga pyramidalis), and the volatile germander (teucrium marum), or the fiery majoram (origanum creticum)?
How can the powers of the verbena (v. officinalis) be said to resemble those of the active hyssop (gratiola officinalis) in the family personatae? How different are the actions of the glycyrrhiza and geoffroya, although in the same family of the papilionaceae!
In the family of the lomentaceae, what parallel exists betwixt the properties of the ceratonia silliqua and those of the fumatory (fumaria officinalis), of the polygala senega and the Peruvian balsam (myroxylon peruiferum)?
Or is there any likeness in properties amongst the nigella sativa, the garden rue (ruta graveolens), the peony (paeonia officinalis), and the cellery-leaved crowfoot (ranunculus sceleratus), although one and all are in the family of the multisiliquae?
The dropwort (spiraea filipendula) and the tormentil (tormentilla erecta) are united in the family senticosae, and yet how different in properties! The red currant (ribes rubrum), and the cherry-laurel (prunus laurocerasus), the rowan (sorbus aucuparia), and the peach (amygdalus pirsica), how different in powers, and yet in the same family of the pomaceae!
The family succulentae unites the wall-pepper (sedum acre) and the portulaca oleracea, certainly not because they resemble each other in effects! How is it that the stork’s-bill and the purging-flax (linum catharticum), the sorrel (oxalis acetosella), and the quassia (q. amara), are in the same family? Certainly not because their powers are similar!
How various are the medicinal properties of all the members of the family ascyroideae! and of those of the dumosae! and of those of the trihilatae! In the family tricoccae, what has the corrosive spurge (euphorbia officinalis) in common with the box (buxus sempervirens), which has such a decided influence on the nervous system?
The tasteless rupture-wort (herniaria glabra), the acrid phytolacca decandra, the refreshing goosefoot (chenopodium ambrosioides), and the biting persicaria (polygonum hydropiper), what a motley company in the family oleraceae! How dissimilar in action are the scabridae!
What business has the mild, slimy, white lily (lilium candidum) beside the garlic (allium sativum), or the squill (scilla maritima); what the asparagus (a. officinalis) beside the poisonous white hellebore (veratrum album), in the family liliaceae?
I am far from denying, however, the many important hints the natural system may afford to the philosophical student of the materia medica and to him who feels it his duty to discover new medicinal agents; but these hints can only help to confirm and serve as a commentary to facts already known, or in the case of untried plants they may give rise to hypothetical conjectures, which are, however, far from approaching even to probability.
But how can a perfect similarity of action be expected amongst groups of plants, which are only arranged in the so-called natural system, on account of often slight external similarity, when even plants that are much more nearly connected, plants of one and the same genus, are sometimes so different in their medicinal effects.
Examples of this are seen in the species of the genera impatiens, serapias, cystisus, ranunculus, calamus, hibiscus, prunus, sedum, cassia, polygonum, convallaria, linum, rhus, seseli, coriandrum, aethusa, sium, angelica, chenopodium, asclepias, solanum, lolium, allium, rhamnus, amygdalus, rubus, delphinium, sisymbrium, polygala, teucrium, vaccinium, cucumis, apium, pimpinella, anethum, seandia, valeriana, anthemis, artemisia, centaurea, juniperus, brassica.
What a difference betwixt the tasteless tinder amadou (boletus igniarius) and the bitter, drastic boletus laricis; betwixt the mushroom (agaricus deliciosus) and the agaric (agaricus muscarius); betwixt the woody stone moss (lichen saxatilis) and the powerful Iceland moss (lichen Islandicus!)
Though I readily admit that, in general, similarity of action will be much oftener met with betwixt species of one genus, than betwixt whole groups of families in the natural system, and that an inference drawn from the former will have a much greater degree of probability attaching to it, than one from the latter; yet my conviction compels me to give this warning, that, be the number of genera ever so many whose species resemble each other very much in their effects, the lesser number of very differently acting species should make us distrustful of this mode of drawing inferences, since we have not here to do with mechanical experiments, but that most important and difficult concern of mankind-health.
