One of the examples that the Gemara discusses of things that are assur behana’ah – that one cannot derive pleasure or benefit from – is kodashim – things consecrated for use in the Temple. The technical term for deriving such pleasure is me’ila – making inappropriate use of sacred property.
A baraita is brought by the Gemara quoting Rabbi Shimon ben Pazi in the name of Rabbi Yehoshua ben Levi who teaches that not all types of benefit are considered me’ila. Specifically, sound (e.g. hearing the singing of the Levites), sight (e.g. viewing the beauty of the Temple itself or even making use of the light from the Temple for mundane activities) and smell (e.g. enjoying the smell of the ketoret – the Temple incense) are not considered making inappropriate use of the Temple’s property.
The Gemara questions this ruling by pointing out that the Torah prohibits one from making ketoret for his own use (Shemot 30:37), and that someone who smells the ketoret that was made for use in the Temple may not be held liable for it, but nevertheless is considered to have committed me’ila.
Rather, Rav Pappa said: Sound and sight are not subject to the prohibition of misuse of consecrated property, because they have no substance.
Therefore, smelling the ketoret would be considered me’ila until after it had been lit, that is to say, until after the mitzva had been fulfilled. The distinction that Rav Pappa makes is based on the fact that the properties of both smell and taste are carried by tiny bits of the substance itself that act upon receptors in the person’s nose and mouth. Thus, the senses of smelling and tasting derive benefit from the object itself. In order to see and hear, however, the body has receptors that react to light waves or sound waves that come from an object, but are not part of the object itself.