1) One may fulfill his obligation by wearing tefillin that he borrowed from his fellow; one may even recite a berachah when wearing borrowed tefillin.[79]

2) Generally speaking, one may take another person’s tefillin without his knowledge and wear it for the mitzvah, as he may presume that the owner would agree to have a mitzvah performed with his property (“נוח לו לאדם שתיעשה מצוה בממונו”).[80]

a. In such a case, the borrower must ensure to return the tefillin after using it in the manner in which it was wrapped originally. Otherwise, he would be considered as having stolen the tefillin, and would not be considered as having fulfilled the mitzvah; and the berachah he recited would be deemed a berachah le’vatalah (a berachah recited in vain).[81]

b. If the tefillin’s owner is known to forbid others from using his tefillin, such as if he finds it loathsome to wear articles that others have worn, or if he customarily locks his tefillin in a chest, then one may not take his tefillin without his permission.[82] 

c. A God-fearing person should refrain from taking other people’s tefillin without their permission, since nowadays many people refuse to allow others to use their possessions, including their tefillin. If there is any uncertainty as to whether the owner could be presumed to allow others to use his tefillin, one may not take the tefillin as he would thereby run the risk of transgressing the prohibition against theft. Additionally, the berachah he recites over the tefillin could potentially be a berachah le’vatalah.[83]

3) One does not fulfill the mitzvah by wearing stolen tefillin, and if one wears stolen tefillin he may not recite a berachah.[84]