The derasha that blind people are not valid witnesses

Summary: Found in the Tur, with a basis and derasha in Tosefta Shevuos.

Post: In parashat Vayikra, in Vayikra 5:1:

1. If a person sins, whereby he accepts an oath, and he is a witness [to some matter] by seeing or knowing [it], yet he does not testify, he shall bear his transgression;

א. וְנֶפֶשׁ כִּי תֶחֱטָא וְשָׁמְעָה קוֹל אָלָה וְהוּא עֵד אוֹ רָאָה אוֹ יָדָע אִם לוֹא יַגִּיד וְנָשָׂא עֲוֹנוֹ:

In Torat HaTur, he cites the following Tur:

The Tur in Choshen Mishpat siman 35 writes: "A blind man in invalid, though he recognizes the voice and the people, and directs his testimony, the Torah invalidates him, for it is written וְהוּא עֵד אוֹ רָאָה אוֹ יָדָע."

Though one might interpret this as "he is a witness, either by seeing it or by knowing it in some other way", it seems like the midrash here understands this pasuk otherwise. How? Is it simply ignoring the word אוֹ? Is it employing selective citation, to sever off  אוֹ יָדָע? Maybe not. We can simply say that the Torah states וְהוּא עֵד, and such an eid needs to be capable of knowledge via direct sight or some other method. And since sight is not one of the two options for the blind, they are not within the realm of valid witnesses. Peshat and derash both work here in tandem.

As discussed in footnote 258 on the page (see image above):

"In Torah Temima, os 18, he shows that this din is explicit in the Tosefta in Shevuos perek 3, [the very last brayta in the perek]:

ג,ו  והוא עד הכשר לעדות (ויקרא ה) ושמעה להוציא את החרש.  או ראה להוציא את הסומא או ידע להוציא את השוטה אם לא יגיד ונשא את עונו להוציא את האלם. 

[where it excludes, based on phrases throughout this verse, the deaf, blind, imbeciles, and mute.]

And it is confounding according to this what the Kesef Mishneh [=Rav Yosef Karo] wrote at the end of the second perek of hilchos eidus [in the Rambam], that he wrote 'where is this derasha found?', and behold, it is an explicit Tosefta.

And see Pischei Teshuva, seif katan 7, who precedes him in pointing out this source in the aforementioned Tosefta."

I don't think that it is so astonishing. It was not necessarily the case (can we establish this one way or the other based on citations?) that Rav Yosef Karo had access to the Tosefta on every masechet.