A Good Rebuttal -Parshas Shelach

The Brisker Rav said that his father R. Chaim  once told him
that the שפת אמת האט אים גוט גיענפערט (The S.E. answered
him back with a strong rebuttal.)

The Radziner Rebbe was trying to promote his תכלת (blue ציצית)
Most Gedolim didn't accept his arguments and weren't convinced
it to be the real תכלת.

The Radziner claimed that even those who are not sure if it is the
real תכלת should also wear them because of ספק דאורייתא לחומרא.

The שפת אמת answered him that the Halacha of ספק דאורייתא לחומרא applies  only when by your action or inaction you are definitely מקיים the מצוה or not עובר the עבירה

eg: You are not sure if you Bentched (ברכת המזון (דאורייתא. You need
to be Machmir and Bentch. By doing so you will have definitely מקיים the מצוה of ברכת המזון. Therefore we apply the Halacha of ספק דאורייתא לחומרא

However, in the case where even after you are Machmir and apply
the so-called  תכלת you are still not sure to have been מקיים the מצוה
of  תכלת, there we don't apply the Halacha of ספק דאורייתא לחומרא.