Our Gemara on Amud Beis states the famous halakhic principle that one cannot make another person’s possession forbidden. For example, if I declare an object of my friend’s to be my god and object of worship and bow to it, it does not become a forbidden object that must be destroyed, unlike if the owner had done that. A person has no authority over another person’s possessions.
Tosafos (“Ein”) raises a question about the applicability of this principle. He says, if so, why can one make someone else’s food treif by mixing in Unkosher substances? Tosafos’ answer is to make a distinction between an action and a thought. When the activation of the prohibition is dependent on action alone, then even someone else can activate the prohibition. The prohibition is not based on anything internal and thus the criteria is simply the deed and not dependent on who owns it. You might say, the milk was factually spilled into the meat, and there is no point in crying over it. This is in distinction to Kilayim, mixing growths in a vineyard (Mishna Kilayim 5:6), which depends on intention to have them both grow. The fact that they are growing alone is not enough. Therefore, someone else cannot ruin your vineyard by planting wheat in it, unless you also desire it.
It is an interesting distinction between the forbidden plantings and forbidden foods. In regard to food, it is the actual presence of the substance that activates the prohibition while in the case of forbidden planting mixtures the presence of two different ingredients is not the problem unless there is intent. This leads us to conclude there is something conceptually different about non-kosher foods and forbidden mixing of species. It seems non kosher foods have an intrinsic noxious and damaging quality. Indeed there is this concept called timtum Halev, a spiritual obstruction in the heart (see for example, Shulkhan Arukh YD 81:17 and Shach (YD 81:26) that comes from non kosher food. While it may have spiritual effects, it seems to be rooted in a physical catalyst. Much as wine, which is a chemical, certainly affects our spiritual and emotional states, which are not entirely physical. However, forbidden mixtures of plantings are not intrinsically problematic, rather the intentional act of mixing and subverting the original biological blueprints and patterns is the problem. This is also why mixed produce is not forbidden for consumption if done by others without your assent (see Shulkhan Arukh YD 296:4-5). It is not fact dependent but intention dependent.
This offers a fascinating window into the reasons for the commandments. We see from here that some prohibitions have to do with a state of mind and sensitivity or regard for sanctity, while others may be based on actual physical toxicities that the Torah wants us to avoid.
Translations Courtesy of Sefaria, (except when, sometimes, I disagree with the translation .)