Spirit
Mind
noun
せいしん
精神 is both the religious and non-religious Spirit.
They say せい Shintoism しん is all about Spirits.
The latest sensation senせいしん (fad) really awoke his Spirit.
Calm your Spirit. せいしん sounds calming because it sounds like satiate せいし(ん). If you're full (satiated), then your Spirit will feel at ease.