Chasidut zu Tehillim 133:2

And this is the aspect of (Ex. 13), “Sanctify to Me all the male firstborns, whatsoever opens the womb among the children of Israel,” which is the beginning of the first passage in the tefillin. For the essence of the tefillin are drawn from the sanctity of the firstborn, which is the first birth, which is the aspect of Yetziath Mitzrayim, which is the aspect of birth, as is known, as on account of this we need to sanctify the first birth which is a male firstborn. As written there, “And it came to pass, when Par`oh would hardly let us go... therefore I sacrifice” etc. And all this is in order to draw on oneself the sanctity of birth of the brains which is the essence of birth, that is, to manage to renew ones vitality and brains at all times as if he was born today, as mentioned. Which, this is is the essence of long life, which are the aspect of tefillin as mentioned. And therefore one needs to give the firstborn to the Kohen or redeem him from him, for the Kohen is the aspect of the elder in holiness, in the aspect of (Ps. 133), “It is like the precious oil upon the head, coming down upon the beard; even Aharon's beard” etc., the aspect of the Eight Tikkunim of the Kohen Gadol, as is known. And this is what our Rabbis z”l said (Shabbath 151), “And let your head lack no oil (Eccl. 9:8) — this is head tefillin.” For the essence of tefillin are drawn from the aspect of “like the precious oil upon the head...” of the elder, Aharon, as mentioned. And therefore by means of giving the firstborn, who is the first birth, to the Kohen, we draw the sanctity of the birth of the brains and vitality, to remember to always renew himself as if he was born today, which this is the essence of tefillin, which are the aspect of brains and long life which we receive from the aforementioned Elder in Holiness, who said that he is extremely old and yet has not begun living at all, as if he was born today. For, one needs to each time begin anew as mentioned, and this is the aspect of Mashiach, of whom it is said (Ps. 2), “Today I have borne you.” For Mashiach will attain this aspect perfectly, which is the aforementioned aspect of long life, as each moment he will begin living anew as if he was born today, in the aspect of “I have borne you today.” For Mashiach will attain the aspect of above time, as explained in Rabbeinu z”l's words on this verse, “I have borne you today;” see there [LM II #61]. For this aspect that the blind one boasted of, who is old yet infantile etc. and the whole world does not amount to him so much as an eyeblink etc., all this is the aspect of above time, which this is the aspect of long life that Mashiach will attain, the aspect of (Ps. 21), “He asked life of You; You gave it to him,” which is the aspect of “David King of Yisrael is Alive and Well.” For David is Mashiach. And this is the aspect of tefillin; that is where is the root of the Mashiach's soul, as is brought, as the root of Mashiach's kingship is in the aspect of the tefillin's knot, and as Rabbeinu z”l said (LM #54), which this is the aspect of (Sam. 1 25), “yet the soul of my lord shall be bound in the bundle of life,” which is said of David, who is Mashiach. The bundle of life is the aspect of tefillin knot, which are the aspect of life as mentioned, the aforementioned aspect of long life, as mentioned.