The Gemara on Daf 62a Still this week) says that according to Ulah, Rav Meir must hold that Tefilin is "mitzvas aseh she'ein hazman grama" So why in Brachos on Daf 20b when the mishna says women are patur from Tefilin the gemara asks pshita?
daniel, jerusalem, israel
A good Kashya. In fact, Tosfos (cited by the Gilyon ha'Shas) asks it in Eruvin.
Tosfos answers that, since those who define Tefilin as a Mitzvas Asei she'ha'Z'man G'rama, consider it obvious, it would not have sufficed to answer that it comes to preclude R. Meir (and R. Yehudah), which it could have done (in answer to your question). The Gemara found it necessary to go further, and to find a reason as to why they might have been Chayav.
Rav Eliezer Chrysler