(Exodus 13:10) commands, "And you shall keep this statute at its appointed time." The word "statute" — chukkah — could theoretically refer to any number of commandments. Perhaps it means all the mitzvot (commandments) collectively, a general instruction to observe God's laws. The Mekhilta asked: what exactly does "this statute" refer to?
The answer is determined by context. The Mekhilta applied a straightforward interpretive principle: when a verse uses a demonstrative pronoun — "this" — it points to the subject currently under discussion. And the subject immediately preceding this verse is tefillin (leather phylacteries worn during prayer). The preceding passages detail the laws of tefillin — what they contain, where they are placed, the order of donning them. "This statute" therefore means the statute of tefillin specifically, not the commandments in general.
The ruling might seem obvious, but in the rabbinic method, nothing is assumed. A word like "this" could be vague, referring backward to one subject or forward to another. The Mekhilta anchors it firmly to its immediate context.
The phrase "at its appointed time" adds a further dimension. Tefillin are a time-bound commandment — they are worn during the day but not at night, on weekdays but not on Shabbat (the Sabbath) or festivals (according to most authorities). The verse does not merely command observance. It commands observance at the right time. Even a mitzvah as sacred as tefillin must be performed in its proper season, not indiscriminately.