Four gateways to liberation 四解脫門（空、無相、無願、無作）

According to Sutra there are three gateways to liberation, whereas according to Vajrayana there are four gateways (Tib. རྣམ་པར་ཐར་པའི་སྒོ་བཞི།, Wyl. rnam par thar pa’i sgo bzhi) or four types of understanding through which one can approach realization and hence liberation. They are:

1. emptiness (Skt. śūnyatā; Tib. སྟོང་པ་ཉིད།, Wyl. stong pa nyid)
2. absence of characteristics or attributes; signlessness (Skt. animitta; Tib. མཚན་ཉིད་མེད་པ།, Wyl. mtshan nyid med pa)
3. wishlessness; absence of expectancy in terms of result (Skt. apraṇihita; Tib. སྨོན་པ་མེད་པ།, Wyl. smon pa med pa)
4. the ultimate emptiness or lack of composition of all phenomena.

In terms of symbolism the four doors of a traditional Tibetan temple are said to symbolize the four gateways to liberation and act as a reminder that if we want to attain permanent liberation from suffering we need to enter the path that leads to liberation.