As Quoted in The Gerorgian Times in 2008 http://www.geotimes.ge/index.php?m=home&newsid=12354.eng
“Let any one try, I will not say to arrest, but to notice or attend to, the present moment of time. One of the most baffling experiences occurs. Where is it, this present? It has melted in our grasp, fled ere we could touch it, gone in the instant of becoming.”
Source: 1890s, The Principles of Psychology (1890), Ch. 15
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William James 246
American philosopher, psychologist, and pragmatist 1842–1910Related quotes

Variant: Life can be found only in the present moment. The past is gone, the future is not yet here, and if we do not go back to ourselves in the present moment, we cannot be in touch with life.

“The past has ended its time, the present is the moment, the future the becoming.”
Original: (it) Il passato ha concluso il suo tempo, il presente è l'attimo, il futuro il divenire.
Source: prevale.net

“There is only the present. A painting is an instant of time that has escaped oblivion.”
1970's, Conversations with Samuel Beckett and Bram van Velde (1970 - 1972)

Winter, § 24, p. 287; in Conducting Effective Faculty Meetings (2008) by Sue Ellen Brandenburg, p. 12 this appears paraphrased in the form: "Time is money says the proverb, but turn it around and you get a precious truth. Money is time."
The Private Papers of Henry Ryecroft (1903)
Context: Time is money — says the vulgarest saw known to any age or people. Turn it round about, and you get a precious truth —money is time. I think of it on these dark, mist-blinded mornings, as I come down to find a glorious fire crackling and leaping in my study. Suppose I were so poor that I could not afford that heartsome blaze, how different the whole day would be! Have I not lost many and many a day of my life for lack of the material comfort which was necessary to put my mind in tune? Money is time. With money I buy for cheerful use the hours which otherwise would not in any sense be mine; nay, which would make me their miserable bondsman. Money is time, and, heaven be thanked, there needs so little of it for this sort of purchase. He who has overmuch is wont to be as badly off in regard to the true use of money, as he who has not enough. What are we doing all our lives but purchasing, or trying to purchase, time? And most of us, having grasped it with one hand, throw it away with the other.

“Let me say it again:
the present moment is all you ever have.”
Variant: Realize deeply that the present moment is all you will ever have.
Source: A New Earth: Awakening to Your Life's Purpose

“Love can touch us one time
And last for a life time
And never let go till
We're gone.”