Using someone’s item without permission

Using someone’s item without permission:[1]

It is forbidden for one to use an item that belongs to one’s friend without his permission.[2] This applies even if one only plans to use it on a mere occasion.[3] It is thus forbidden for one to visit his friend’s house in order to use an item of his without his permission even on mere occasion.
One knows for certain that the owner is not particular:[4] If one knows for certain that the owner is not particular against one using his item, then it is permitted to use it without his explicit permission.
Items that people are not generally particular against being used:[5] Those items in which it is not common at all for any people to be particular against someone using without their permission, such as items that do not receive any loss or damage at all through being used, it is permitted to use these items without permission. However, those items which minorities of peoples are particular against them being used by others due to worry of damage, then it is forbidden to be used without permission even if majority of people are not particular.[6] This applies even if one is positive that no damage will occur to the item at all, it is nevertheless forbidden to borrow without permission.[7]

Summary:
It is forbidden to use someone else’s item without permission, unless one is certain the person is not particular against one doing so, or it is not common at all for any person to be particular.

Q&A

May one use someone’s pen without permission?[8]
If the pen is inexpensive and was left around in the open, such as in a Beis Midrash and the like, seemingly it may be used, as it can be assumed that it was left there for this purpose.

May one use someone’s phone without permission?
No, unless one is certain the person does not mind him using it.

May one borrow a piece of paper from a friend without permission?
No, unless one is certain the person does not mind him using it.

May one use a hotel’s swimming pool without permission?
No.

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[1] Admur O.C. 14/10 and C.M. Metzia Upikadon 27 regarding Sefarim; Sheila Vesichirus 5 that it goes without saying that this applies to other items of Reshus; Rama O.C. 14/4; Smeh C.M. 75/6; Nimukei Yosef  Tzitzis 12a in name of Ritva; See Piskeiy Teshuvos 14/10

[2] The reason: As the owner of the object suspects that perhaps damage will occur to it, therefore he is not pleased with allowing someone else to use it at all. [Admur ibid; Nimukei Yosef  Tzitzis in name of Ritva]

[3] Admur ibid; M”A 14/10

[4] Admur Sheila Vesichirus 5; Rama C.M. 292/20 regarding Pikadon of Sefarim by Talmid Chachaim; Smeh 292/45

[5] Admur Sheila Vesichirus 5; Shita Mekubetzes Baba Metzia 22b

[6] The reason: Although majority of people are not particular due to that chance of damage is very rare, nevertheless, we do not follow the majority in this regard to say that one can assume that this owner is not particular against it being used. [Admur ibid]

[7] Admur ibid and C.M. Metzia Upikadon 27 in parentheses regarding Sefarim
The reason: As nevertheless, the owner of the item suspects that perhaps it may get damaged, and thus if he knew that this person was using his item, it is possible that he would be particular, and it is thus found that he has borrowed without permission of the owners [Sheol Shelo Midaas], which is considered stealing, even if one causes no damage or loss to the item at all, as explained in Hilchos Metzia Upikadon 27. [Admur ibid; Metzia Upikadon ibid in parentheses]

[8] See Gr”a C.M. 163; Minchas Yitzchak 7/130; Piskeiy Teshuvos 14/10; See Chesed Lealafim 14/5, brought in Kaf Hachaim 14/31, for a similar defense regarding Siddurim; Lehoros Nasan 6/127-128