The patients generally call attention to the commonest things, while it is strange and peculiar things that guide to a remedy. 
When the physician comes to look over the record after an examination to get the image to classify and arrange it, he will find what is peculiar, and those symptoms that are most general, and those that are but common. These three grades appear in every complete case, and in every complete proving of a remedy. …. Every case has common symptoms, but peculiar symptoms may be absent and you must not expect to cure when peculiar symptoms are absent. 
It is almost impossible to name all the peculiarities in all the cases that
we might find, but there are certain ones that stand out. First of all, those
symptoms that are common and found in almost all diseases may be left out of our
count unless they manifest themselves in a striking manner. The same may be true
of those ailments and symptoms that usually appear in the disease under
consideration, unless they should be distinguished by some rare peculiarity, and
in this way offer something particularly characteristic, like thirstless fever,
or chill with desire to uncover. On the other hand, all the attendant symptoms
should be carefully noted:
(a) Those symptoms which rarely appear in connection with the leading disease, and therefore are found rarely among the provings.
(b) Those which belong to another sphere of disease than the principal ailment.
(c) Those which have more or less characteristic signs of the medicine even though they have not before been noticed in the present relationship or diagnostic group. 
It is necessary to have individualizing characteristics to enable you to classify that which you have, to perceive the value of symptoms, and, if you must settle down to a few remedies, to ascertain which of these is more important than another, or most important of all. 
The things that characterize are things to make you hesitate, to make you meditate. 
We shall at once see that the common symptoms are those that appear in all cases of measles, that you would expect to find in measles. It would be strange to have measles without any rash; that would be peculiar. 
The things that are general are the first in importance. After these have been gathered, you may go on taking up each organ, and ascertaining what is true of each organ. Many times you will find that the modalities of each organ conform to the generals. Sometimes, however, there may be modalities of the organs, which are particular that are opposed to the general. Hence we find in remedies that appear to have in one subject one thing, and in another subject the very opposite of that thing. In one it will be a general, and in another it will be a particular. 
[Having chosen a remedy according to generals} there may be a lot of little particulars that may appear to contra-indicate, but they cannot; for nothing in particulars can contra-indicate generals. One strong general can over-rule all the particulars you can gather up. Aggravation from heat will throw out Arsenicum from consideration in any case. 
Dr. Hering in his quaint fashion years before the keynote system was ever heard of, said: Every stool must have at least three legs, if it is to stand alone. He advised selecting at least three characteristic symptoms as the basis of prescribing. 
1. Roberts, Herbert A., M.D., The Principles and Art of Cure by Homeopathy.
2. Kent, James Tyler, M.D., Lectures on Homeopathic Philosophy
3. Close, Stuart, M.D., The Genius of Homeopathy
A woman complains of a common symptom such as a dragging down feeling in her bowels in prolapsus. There is nothing about this that will let you find the remedy. A rubric that has more than a dozen remedies will be a common symptom.
The whole problem must be followed from generals to particulars.
How should we decide to give Sepia when Lil tig., Murex., Bell., Puls., Nux-v. and Nat-m will fit?
What is in the Sepia patient that no other patient has? The dragging down is just the same, but with it an awful all-gone sinking feeling in the stomach, and she gets relief only when sitting with the legs crossed. She has a constant feeling of a lump in the rectum that makes her want to pass a stool, but she goes for days without any urging at all. She is sallow and sickly, talks of bilious symptoms and has a yellow saddle over the nose. She has an aversion to children, feels very sad that she does not love her husband as she ought to, and is unable to exercise the love she has to her children.
What would a Nux-v patient say about this? She would be chilly, full of coryza, with stuffing up of the nose in a warm room, she would be very irritable, snappish, want to kill somebody, want to throw her child in the fire, want to kill her husband, have constipation probably which with every pain would make her want to go again, little stool is passed and urging to go frequently.