Commentary for Bava Metzia 3:6
וליתני אני מצאתיה ואנא ידענא דכולה שלי אי תנא אני מצאתיה הוה אמינא מאי מצאתיה ראיתיה אע"ג דלא אתאי לידיה בראיה בעלמא קני תנא כולה שלי דבראיה לא קני
means 'thou hast taken hold of it'? — It is admitted that the Scriptural use of the term 'found' implies having taken hold, but the Tanna uses popular language, in which, on seeing something, one might use the term 'found it', [the belief being prevalent] that one acquires [a lost article] by sight alone. For this reason it was necessary to add the plea 'IT IS ALL MINE' and thus to indicate that the mere seeing [of an ownerless object] constitutes no claim to possession. But even so, would it not have been sufficient to state 'IT IS ALL MINE' without the plea of 'I FOUND IT'? — Had [the Mishnah] stated only the plea 'IT IS ALL MINE' I might have said that elsewhere [in the Talmud] the term 'found' is used to mean ['seen', and the conclusion would have been drawn] that mere sight constitutes a claim to possession. For this reason the Mishnah states first 'I FOUND IT' and then 'IT IS ALL MINE' so that we may gather from the additional clause that mere sight does not constitute a claim to possession.
Rashi on Bava Metzia
Tosafot on Bava Metzia
Tosfos will quote two Mishnayos where it is evident that one cannot acquire a lost object by seeing it. If so, why was it necessary for our Mishna to repeat the same ruling?
The Mishna on 9b: Even though the Mishna teaches (below 9b): [Ruvain] who saw a lost object and said to [Shimon] “give it to me”. The Mishna rules that Shimon who was asked to pick it up may acquire it for himself. We see that [Ruvain] did not acquire it by seeing it, since Ruvain who asked Shimon to pick it up, obviously was the first to see it and even so Shimon may lift it up for himself. It is evident that Ruvain did not acquire it by seeing it.
The Mishna on 10a: So too, we learned in another Mishna: [Ruvain] saw a lost object and fell on it, but did not lift it up and Shimon comes and lifts it up, the Mishna teaches us that [Ruvain] did not acquire it since he did not make a proper act of acquisition. But Ruvain did see it first? Once again we see that seeing the lost object is not a way of acquiring it. Why does our Mishna need to stress this lesson?
[The Gemara] could have deflected the proof from those Mishnayos, that seeing is not an act of acquisition, by saying, since [Ruvain] said to Shimon in the Mishna on 9b “give it to me”, or in the Mishna on 10a he fell on it, he is showing his mindset, that he is not satisfied to acquire it by any method of acquisition until it actually reaches his hand. It is only then that those Mishnayos rule that seeing the lost object is not a valid method of acquiring it. However, when one did not in any way indicate that he does not want to acquire the lost object by seeing it, it may very well be that seeing is a proper way of acquiring it. Our Mishna must therefore teach us that even when there is no indication that he is dissatisfied to acquire it by seeing, he cannot acquire it.