While St. Thomas referred to the loadstone in a number of places as something whose nature and occult properties are well known, it was always as an example or as a tangential reference. One does not find a systematic treatment of the loadstone in St. Thomas, but there are enough references to provide a fairly explicit statement of what he considered to be the nature of the magnet.
In one of his earliest writings, St. Thomas argued that the magnet attracts iron because this is a necessary consequence of its nature.[22]
Respondeo dicendum, quod omnibus rebus naturaliter insunt quaedam principia, quibus non solum operationes proprias efficere possunt, sed quibus etiam eas convenientes fini suo reddant, sive sint actiones quae consequantur rem aliquam ex natura sui generis, sive consequantur ex natura speciei, ut magneti competit ferri deorsum ex natura sui generis, et attrahere ferrum ex natura speciei. Sicut autem in rebus agentibus ex necessitate naturae sunt principia actionum ipsae formae, a quibus operationes proprie prodeunt convenientes fini....
Due to its generic form, the loadstone is subject to natural motion of place of up and down. However, the "virtus" of its specific form enabled it to produce another kind of motion—it could draw iron to itself.
Normally the "virtus" of a substance is limited to those contact effects that could be produced by the form operating through the active qualities of one substance, on the relatively passive qualities of another. St. Thomas asserted the loadstone to be one of these minerals, the occult powers of whose form goes beyond those of the prime qualities.[23]
Forma enim elementi non habet aliquam operationem nisi quae fit per qualitates activas et passivas, quae sunt dispositiones materiae corporalis. Forma autem corporis mineralis habet aliquam operationem excedentem qualitates activas et passivas, quae consequitur speciem ex influentia corporis coelestis, ut quod magnes attrahit ferrum, et quod saphirus curat apostema.
That this occult power of the loadstone is a result of the direct influence of the "virtus coeli" was expounded at greater length in his treatise on the soul.[24]
Quod quidem ex propriis formarum operationibus perpendi potest. Formae enim elementorum, quae sint infimae et materiae propinquissime, non habent aliquam operationem excedentem qualitates activas et passivas, ut rarum et densum, et aliae huiusmodi, qui videntur esse materiae dispositiones. Super has autem sunt formae mistorum quae praeter praedictas operationes, habent aliquam operationem consequentem speciem, quam fortiuntur ex corporibus coelestibus; sicut quod magnes attrahit ferrum non propter calorem aut frigiis, aut aliquid huiusmodi; sed ex quadam participatione virtutis coelestis. Super has autem formas sint iterum animae plantarum, quae habent similitudinem non solum ad ipsa corpora coelestia, sed ad motores corporum coelestium, inquantum sunt principia cuiusdam motus, quibusdam seipsa moventibus. Super has autem ulterius sunt animae brutorum, quae similitudinem iam habent ad substantiam moventem coelestia corpora, non solum in operatione qua movent corpora, sed etiam in hoc quod in seipsis cognoscitivae sunt, licet brutorum cognitio sit materialium tantum et materialiter....
St. Thomas placed the form of the magnet and its powers in the hierarchy of forms intermediate between the forms of the inanimate world and the forms of the organic world with its hierarchy of plant, animal and rational souls. The form of the loadstone is then superior to that of iron, which can only act through its active and passive qualities, but inferior to the plant soul, that has the powers of growth from the "virtus coeli." This is similar to Galen's comparison of the magnet's powers to that of the nutritive powers of organic bodies.
In his commentary on Aristotle's Physics, St. Thomas explained how iron is moved to the magnet. It is moved by some quality imparted to the iron by the magnet.[25]