This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
The reason is found in the fact of the bodily resistance which must be overcome by the drug before it can manifest its action. Every living organism is in the constant effort to keep itself intact - automatically and unconsciously to the individual. This protecting sphere of its distinctive life offers resistance to any foreign intruder, which bodily resistance to everything that tends to disturb its health must be overcome first of all before the drug can manifest its peculiar power. The physiological dose of any drug, therefore, must be large enough (1) to overcome the normal body resistance; (2) sufficiently more to produce symptoms.