When defining the amount of food that needs to be eaten – either the amount needed to fulfill a commandment or the amount needed to determine whether a person is liable for punishment for having eaten something that is forbidden – we must look not only at the volume of food, but also at the amount of time during which the act of eating is performed. Generally speaking, in order for an act of eating to be considered significant, a person must consume ke-zayit be-kheday akhilat peras – an amount the size of an olive in the amount of time that it takes to eat half a loaf of bread. Our Gemara posits that this amount is a biblical law.
Tosafot point out that the fact that ke-zayit be-kheday akhilat peras is a biblical law is clear from a Mishna in Massekhet Keritot (12b), but our Gemara is searching for a source that teaches whether we use the same criteria to determine the significance of the act of eating when the kezayit is mixed together with other food, rather than the simple case of eating a kezayit on its own.
One test case is kutah haBavli – a Babylonian dip made with mouldy bread mixed with whey and salt – where we find a disagreement whether you will be held liable for eating it on Pesaḥ. The Gemara points out that this food is unique because it is used only as a condiment, so no one will ever eat an olive’s worth within the allotted time, and if someone does, we do not consider that to be significant, since batlah da’ato etzel kol adam – literally, “His opinion is nullified in the face of everyone else.”