One of the examples of food mentioned in the first Mishna in our perek, which includes a mixture of hametz and is forbidden to eat on the level of a lav – a simple negative prohibition – rather than the more severe hiyyuv karet (excision from the people of Israel), is Babylonian kutah. This pungent dip made of whey, fermented bread and salt was used as a condiment, and was not normally eaten by itself.
The Gemara explains that Babylonian kutah does not contain the requisite ka-zayit (olive-sized amount) of hametz eaten tokh k’dei akhilat pras – within the amount of time that it takes to eat half a loaf – since it is not normally eaten on its own. Were someone to try to eat a significant amount of it by itself, it would not be considered a normal way of eating, since no one eats it in such a manner – batla da’atei etzel kol adam – his intent is nullified when compared with normative behavior.
The rishonim differ as to how to approach a forbidden activity done in an abnormal way. According to Rashi, the concept of batla da’atei etzel kol adam indicates that this behavior is so strange that halakha does not view it as “eating” in the normal sense. Therefore, the person doing it will not be held liable at all. Tosafot argue that even someone who behaves in an abnormal manner will be held liable for forbidden acts that he does. By pointing out that such behavior is abnormal and saying batlah da’atei etzel kol adam, the Gemara is merely arguing that such a case is not considered a serious one by the author of the Mishna, and therefore cannot be included as the basis for interpreting disagreements between opinions of the tanna’im that appear there.