“Spirit is like the wind, in that we can't see it but can see its effects, which are profound.”

Source: My Sister's Keeper

Source: Surfacing

Variant: Love is like the wind, you can't see it but you can feel it.
Source: A Walk to Remember

As quoted in Infinite Potential: The Life and Times of David Bohm by F. David Peat https://books.google.com/books?id=pobZMUmZbAEC&pg=PA322&dq=The+field+of+the+finite+is+all+that+we+can+see,+hear,+touch,+remember,+and+describe.+This+field+is+basically+that+which+is+manifest,+or+tangible.+The+essential+quality+of+the+infinite,+by+contrast,+is+its+subtlety,+its+intangibility.+This+quality+is+conveyed+in+the+word+spirit,+whose+root+meaning+is+%22wind,+or+breath.%22+This+suggests+an+invisible+but+pervasive+energy,+to+which+the+manifest+world+of+the+finite+responds.+This+energy,+or+spirit,+infuses+all+living+beings,+and+without+it+any+organism+must+fall+apart+into+its+constituent+elements.+That+which+is+truly+alive+in+the+living+being+is+this+energy+of+spirit,+and+this+is+never+born+and+never+dies&hl=en&sa=X&ved=0ahUKEwjotZe8m6_TAhWs5oMKHbA4CkMQ6AEIIzAA#v=onepage&q=The%20field%20of%20the%20finite%20is%20all%20that%20we%20can%20see%2C%20hear%2C%20touch%2C%20remember%2C%20and%20describe.%20This%20field%20is%20basically%20that%20which%20is%20manifest%2C%20or%20tangible.%20The%20essential%20quality%20of%20the%20infinite%2C%20by%20contrast%2C%20is%20its%20subtlety%2C%20its%20intangibility.%20This%20quality%20is%20conveyed%20in%20the%20word%20spirit%2C%20whose%20root%20meaning%20is%20%22wind%2C%20or%20breath.%22%20This%20suggests%20an%20invisible%20but%20pervasive%20energy%2C%20to%20which%20the%20manifest%20world%20of%20the%20finite%20responds.%20This%20energy%2C%20or%20spirit%2C%20infuses%20all%20living%20beings%2C%20and%20without%20it%20any%20organism%20must%20fall%20apart%20into%20its%20constituent%20elements.%20That%20which%20is%20truly%20alive%20in%20the%20living%20being%20is%20this%20energy%20of%20spirit%2C%20and%20this%20is%20never%20born%20and%20never%20dies&f=false (1997) page 322, .

Introduction
Sailing the Wine-Dark Sea: Why the Greeks Matter (2003)

The Romance of Aeronautics (1912)

Source: Foxfire: Confessions of a Girl Gang

Source: The Value of Science (1905), Ch. 5: Analysis and Physics
Context: All laws are... deduced from experiment; but to enunciate them, a special language is needful... ordinary language is too poor...
This... is one reason why the physicist can not do without mathematics; it furnishes him the only language he can speak. And a well-made language is no indifferent thing;
... the analyst, who pursues a purely esthetic aim, helps create, just by that, a language more fit to satisfy the physicist.
... law springs from experiment, but not immediately. Experiment is individual, the law deduced from it is general; experiment is only approximate, the law is precise...
In a word, to get the law from experiment, it is necessary to generalize... But how generalize?... in this choice what shall guide us?
It can only be analogy.... What has taught us to know the true profound analogies, those the eyes do not see but reason divines?
It is the mathematical spirit, which disdains matter to cling only to pure form.<!--pp.76-77

Source: I Capture the Castle