“I am doing it
the it I am doing is
the I that is doing it
the I that is doing it is
the it I am doing
it is doing the I that am doing it
I am being done by the it I am doing
it is doing it”

Quoted in "Epistle to Casualanus", XXXVI, section 32, by St. Augustine; translation by J.G. Cunningham
Context: When I am here I do not fast on Saturday; but when I am at Rome I do: whatever church you may come to, conform to its custom, if you would avoid either receiving or giving offense.

Statement published in A Year of Beautiful Thoughts‎ (1902) by Jeanie Ashley Bates Greenough, p. 172, Third statement for June 11. This has often been misattributed to Helen Keller in some published works since at least 1980, perhaps because she somewhere quoted it.
Variant:
I am only one,
But still I am one.
I cannot do everything,
But still I can do something;
And because I cannot do everything,
I will not refuse to do the something that I can do.
The Book of Good Cheer : A Little Bundle of Cheery Thoughts‎ (1909) by Edwin Osgood Grover, p. 28; also in Masterpieces of Religious Verse (1948) by James Dalton Morrison, p. 416, where it is titled "Lend a Hand"

In an interview in the New York Times (16 December 1957), cited in a footnote on page 32 of "Work, Society, and Culture" by Yves Reni Marie Simon, and also in a footnote (in German) on page 360 of "Vita activa oder Vom taetigen Leben" by Hannah Arend (1981)

Variants:

Basic research is when I'm doing what I don't know I'm doing.

Research is what I'm doing when I don't know what I'm doing.

Basic research is what I am doing when I don't know what I am doing.

Edward Everett Hale in a statement published in A Year of Beautiful Thoughts‎ (1902) by Jeanie Ashley Bates Greenough, p. 172; <!-- and perhaps as early as an edition of Ten Times One is Ten (1870) by Hale--> This has been misattributed to Keller in published works since at least 1980. Keller and Hale were good friends, and letters to Hale can be found in her youthful autobiography The Story of My Life (1902). In 1910 Keller dedicated her poem "The Song of the Stone Wall" to Hale who had died in 1909.
Misattributed
Variant: I am only one, but I am one. I can not do everything, but I can do something. I must not fail to do the something that I can do.

Statement published in A Year of Beautiful Thoughts‎ (1902) by Jeanie Ashley Bates Greenough, p. 172, Third statement for June 11. This has often been misattributed to Helen Keller in some published works since at least 1980, perhaps because she somewhere quoted it.
Variant:
I am only one,
But still I am one.
I cannot do everything,
But still I can do something;
And because I cannot do everything,
I will not refuse to do the something that I can do.
The Book of Good Cheer : A Little Bundle of Cheery Thoughts‎ (1909) by Edwin Osgood Grover, p. 28; also in Masterpieces of Religious Verse (1948) by James Dalton Morrison, p. 416, where it is titled "Lend a Hand"
Variant: I am only one, but still I am one. I cannot do everything, but still I can do something; and because I cannot do everything, I will not refuse to do something that I can do.

Green Eggs and Ham (1960)

60 Minutes interview (1998)