[25] St. Thomas Aquinas, _op. cit._ (footnote 19), vol. 2, _Physicorum Aristotelis expositio_, lib. 7, lect. 3, cap. g (Probatur in motu locali quod movens et motum oportet esse simul), p. 97 (quoted in Gilbert, M: p. 104).
About the time that St. Thomas was writing his letter _De operationibus occultis naturae_ to a certain knight, Petrus Peregrinus was writing from a military camp a letter in which he showed how certain relatively new effects could be produced by the loadstone. He was more interested in what he could do with the magnet than in explaining these effects. However, he discussed it at sufficient length for one to find that his explanation of magnetic phenomena was basically similar to that of his contemporary, St. Thomas.
Peregrinus based his discussion of the loadstone upon its nature and analyzed magnetic phenomena in terms of the change of alteration. In magnetic attraction, the nature of the iron is altered by having a new quality impressed upon it,[26] and the loadstone is the agent that makes the iron the same species as the stone.[27]
... Oportet enim quod illud quod iam conversum est ex duobus in unum, sit in eadem specie cum agente; quod non esset, si natura istud impossible eligeret.
This impressed similarity to the agent, Peregrinus realized, is not a pole of the same polarity but one opposite to that of the inducing pole. To produce this effect, the virtue of the stronger agent dominates the weaker patient and impresses the virtue of the stronger on the weaker so that they are made similar.[28]
... In cuius attractione, lapis fortioris virtutis agens est; debilioris vero patiens.
A further instance of alteration occurs in the reversal of polarity of magnetized iron when one brings two similar poles together. Again, the stronger agent dominates the weaker patient and the iron is left with a similarity to the last agent.[29]
... Causa huis est impressio ultimi agentis, confundentis et alterantis virtutem primi.
In this assimilation of the agent to the patient, another effect is produced: the agent not only desires to assimilate the patient to itself, but to unite with it to become one and the same. Speaking of the motion to come together, he says:[30]