(Qa'idah al-Idhn fi al-Shay' Idhn fi Lawazimihi)

It's Meaning: The meaning of this Precept is that when permission has been granted for a thing, which has related benefits, then that [primary] permission is also permission for its related benefits. What is meant by 'related benefits' are matters that do not separate from what they are related to (in this case the authorised thing) conventionally ('urfan). Such as when a person permits another to live in his house, then this permission of usage (bi al-tasarruf) of the house is also permission in relation to use [for example] the water and electricity and other related benefits of the house. Similar is the case when [for example], a person purchases a house then he would be entitled to the drive/path way to it also.

The Source: It is possible to demonstrate the Precept by the following:
I. The Tendency of Wise men: The tendency of Wise men, throughout the world, is in agreement that the related benefit of a thing will enjoy its law totally in every sense (tamaman). They have phrased the Precept as: "one who has the right to a thing has right to its related benefits".
II. Traditions: There is a report transmitted by al-Shaykh al-Tusi, may God have mercy on him, by a chain leading to al-Hasan al-Saffar who wrote to Abu Muhammad (8th Imam, a.s.); "I asked him regarding possession of related benefits of a land which I was buying from [another] man.  The Imam ('a.s.) stated: When a defined land is bought then the owner shall have whatever is within it, God willing. (1)

What the report indicates as regards the related benefits that are attached [to a piece of land is that they] follow the land in its rulings (
hukman).  One such report is the trustworthy narration (mu'tabar) of al-Sukuni from Abu Abdallah ('a.s.) who said that the Prophet (s) decided a case regarding a man who sold a date palm (nakhl) while he exempted the produce of a  group of date palm trees. The Prophet (s) judged [in favour] that he could enter and exit the grove and utilize the extent covered by the date palms. (2)

The entitlement of the buyer, regarding the related benefits of the grove, is the indicative of the meaning of this Precept. The Precept confirms is further strengthened by the reliance of the scholars upon it. As is the case with al-Shahid al-Awwal, Muhammad bin Makki (may God have mercy on him) regarding his ruling of the right to lease a property [initially] taking it on lease from another.  Basing it on the Precept that Permission with regard to a thing is permission with regards to its related benefits.

(1) al-Wasail : Vol. 12, p. 405, Section 29 from the Chapter of Rules of Contracts
(2)al-Wasail : Vol. 12, p. 406, Section 30 from the Chapter of Rules of Contracts