Impurity, Treife, and the Monsey Butcher Scandal
One of the differences between rationalists and mystics is their different views regarding metaphysical entities. Rationalists minimize the number of supernatural entities and forces; mystics, on the other hand, tend to maximize the number of supernatural entities and forces.
In the mystical worldview, metaphysical entities are not restricted to the world of angels and demons. Rather, there are metaphysical forces which lie at the core of basic principles of Judaism. Rationalists, on the other hand, understand these principles to be independent of a metaphysical framework, the existence of which they do not accept.
The concept of tumah, usually translated as "impurity," lies at the heart of many commandments. Corpses and dead vermin transmit impurity. The emission of certain bodily fluids renders one in a state of impurity, as does childbirth. Animals that are forbidden from consumption are also described as "impure" (the colloquial term "treife" is inaccurate).
But what is this "impurity"? According the mystical approach, this impurity is a metaphysical state—a sort of invisible, spiritual "dirt." It has objective existence, regardless of how people relate to it. According to the rationalist approach, on the other hand, impurity is not a metaphysical state. Rather, it is a designation, a state which we (following God's instructions) ascribe to certain people, creatures and objects. And we are forbidden to eat certain foods, in order to accomplish various functions relating to perfecting our characters and our society; there is nothing inherently metaphysically impure about these foods.
The differing views on the nature of impurity are also seen in the laws of accidental and forced transgression. The Torah is explicit that if someone sins by accident, they must nevertheless bring an offering. But why? Ramban, following the mystical approach, explains that even though it was an accident, the impurity has nevertheless tainted one's soul. Rambam, on the other hand, explains that he has to atone for being somewhat negligent, as had he been more careful, the accident would not have happened.
What if someone ate impure food through absolutely no negligence on their behalf, or in a case where it was halachically required (such as for survival)? According to Rambam, there are no negative consequences whatsoever. Indeed, in a situation where kosher food items are mixed up with a non-kosher food item, according to many views one is entitled to rely on the majority of items and eat all the food, because with any given item, the odds are that it is kosher. This is despite the fact that if one eats all the items, one has certainly consumed the non-kosher item! Evidently, there is no concern for metaphysical harm; the only problem of eating non-kosher food is transgressing institutional prohibition.
According to the mystical approach, on the other hand, even if one eats non-kosher food in a situation where it is halachically permissible, or where there was no negligence at all, it has nevertheless still harmed one's soul. This was seen in the discussion following the notorious scandal in Monsey, where a respected butcher was discovered to have been selling non-kosher meat for years. Responsa published after the event made no reference to Rambam's view (they were probably unaware of it) and quoted numerous opinions from the mystically-inclined Rishonim to demonstrate that even though it was a case of onnes (no negligence at all), there would still be metaphysical harm caused to peoples' souls. (The few opinions quoted in opposition stated that the metaphysical harmful characteristics of non-kosher food only come into existence where eating them is a sin, not in a case of onnes, but this is far from straightforward. The Rishonim who advocate for non-kosher food possessing metaphysically harmful qualities certainly appear to see this as a property of the food itself, which is the very cause of it being halachically forbidden, rather than being generated as a consequence of the prohibition.)
The differing views of the nature of non-kosher food may also explain the differing approaches with regard to checking vegetables for insects. Those who advocate for a far more intensive search than was traditionally done are usually of the mystical mindset, and are concerned for actual metaphysical harm that will be sustained by eating insects. Those following a rationalist approach, on the other hand, are of the view that following the classical halachic requirements is all that is required, and there is no metaphysical harm about which to be concerned.
 See Mishneh Torah, Hilchos Mikvaos 11:12. For extensive discussion, see Menachem Kellner, Maimonides' Confrontation with Mysticism, chapter 4.
 See Kellner, Maimonides' Confrontation with Mysticism, chapter 2.
 Ramban, Commentary to Leviticus 4:2.
 Rambam, Guide for the Perplexed 3:41.
 Guide 3:41; Mishneh Torah, Hilchos Maachalos Issuros 14:10-13. In fact it would seem that Rambam would define the category of onnes as a case where there was no negligence, whereas others would have a more limited definition.
 See Rabbi Gedaliah Oberlander, "Timtum Halev MeiAchilas Ma'achalos Issuros B'Onnes," Ohr Yisrael 45 pp. 103-109, and the rejoinder by Rabbi Yaakov Dovid Luban, Ohr Yisrael 46, pp. 49-52, available online here.