Bava Batra 94B

אימא סיפא ⁠– רבי יוסי אומר: יבור.

say the latter clause of the Mishnah: R' Yosei says: It is insufficient to merely reduce the quantity of seeds of a different kind; rather, one must pick out all the seeds of a different kind.

אי אמרת בשלמא כיותר מרובע טנופת דמי, בהא קא מיפלגי ⁠– תנא קמא סבר: לא קנסינן התירא אטו איסורא, ורבי יוסי סבר: קנסינן. אלא אי אמרת כרובע דמי, אמאי יבור?

The Talmud explains the difficulty: Granted, if you say that 1/4th-kav of seeds of a different kind per se’a with regard to diverse kinds is comparable to more than 1/4th-kav of impurities per se’a with regard to a sale, then it is about this that they disagree: The first tanna holds that we do not penalize one by requiring him to remove that which is permitted due to that which is prohibited, and consequently it is sufficient to merely reduce the quantity of seeds of a different kind to an acceptable level, whereas R' Yosei holds that we do penalize one by requiring him to remove that which is permitted due to that which is prohibited. But if you say that 1/4th-kav of seeds of a different kind per se’a with regard to diverse kinds is comparable to 1/4th-kav of impurities per se’a with regard to a sale, then why does R' Yosei rule that one must pick out all the seeds of a different kind?

התם היינו טעמא דרבי יוסי ⁠– משום דמיחזי כי מקיים כלאים.

The Talmud answers: There, with regard to diverse kinds, this is the reasoning of R' Yosei: One must pick out all the seeds, because once he is purifying the admixture of seeds, if he deliberately leaves a quantity of seeds of a different kind mixed in, it appears as though he is intentionally planting and maintaining diverse kinds in his field.