This is Marie Kondo's famous criterion whether to keep something when tidying up, presented in Book - The life-changing magic of tidying up.. the Japanese art of decluttering and organizing: When touching this item, does it spark joy?
Decluttering is as simple, as picking up every single item you own and asking "does this spark joy?" If yes, keep it, otherwise discard it.
Joy can be defined in many ways, as Kondo puts it:
A simple design that puts you at ease, a high degree of functionality that makes life simpler, a sense of rightness, or the recognition that a possession is useful in our daily lives—these, too, indicate joy.
If you still struggle with the concept of joy, the best way to feel joy is to compare. Take an item that is truly dear to you, hold it in your hand, and compare the feeling you get to the feelings emitted by other items.
This process of being honest with oneself on what really sparks joy is a form of self discovery, which is why Tidying fully is life transforming
If you are scared that nothing will be left, Kondo argues:
The real tragedy is to live your entire life without anything that brings you joy and never even realize it.
It is important that you really take our all of the items and take them into your hand. When they are stored away, our judgement whether they spark joy is clouded, Kondo claims. (There could be research backing this one up, see with how we treat anonymized people)
(There are very few exceptions to this rule, such as insurance papers and receipts)