“Scientists dream about doing great things. Engineers do them.”

Source: Into the Wild

as quoted by Gordon Shrum. In an article by Robert Craig Brown, The life of Sir John Cunningham McLennan http://www.physics.utoronto.ca/overview/history/mclennan, Physics in Canada, March / April 2000.

Letter to his father, expressing his desire to study physics instead of mechanical engineering, as his father had wanted. His father agreed, on the condition that Bhabha pass the Mechanical Engineering Tripos with first class. He did that, and later passed the mathematical Tripos with First class too, as quoted in the "Homi Jehangir Bhabha" profile at the Vigyan Prasar Science Portal.
Context: I seriously say to you that business or job as an engineer is not the thing for me. It is totally foreign to my nature and radically opposed to my temperament and opinions. Physics is my line. I know I shall do great things here. For, each man can do best and excel in only that thing of which he is passionately fond, in which he believes, as I do, that he has the ability to do it, that he is in fact born and destined to do it... I am burning with a desire to do physics. I will and must do it sometime. It is my only ambition. I have no desire to be a "successful" man or the head of a big firm. There are intelligent people who like that and let them do it. … It is no use saying to Beethoven "You must be a scientist bybye for it is great thing" when he did not care two hoots for science; or to Socrates "Be an engineer; it is work of intelligent man." It is not in the nature of things. I therefore earnestly implore you to let me do physics.

Source: The Butterfly as Companion: Meditations on the First Three Chapters of the Chuang-Tzu
Context: How do I know that enjoying life is not a delusion? How do I know that in hating death we are not like people who got lost in early childhood and do not know the way home? Lady Li was the child of a border guard in Ai. When first captured by the state of Jin, she wept so much her clothes were soaked. But after she entered the palace, shared the king's bed, and dined on the finest meats, she regretted her tears. How do I know that the dead do not regret their previous longing for life? One who dreams of drinking wine may in the morning weep; one who dreams weeping may in the morning go out to hunt. During our dreams we do not know we are dreaming. We may even dream of interpreting a dream. Only on waking do we know it was a dream. Only after the great awakening will we realize that this is the great dream. And yet fools think they are awake, presuming to know that they are rulers or herdsmen. How dense! You and Confucius are both dreaming, and I who say you are a dream am also a dream. Such is my tale. It will probably be called preposterous, but after ten thousand generations there may be a great sage who will be able to explain it, a trivial interval equivalent to the passage from morning to night.

TED Talk, "From Mach-20 glider to Hummingbird Drone" (March 2012); also in Counterterrorism and Cybersecurity: Total Information Awareness (2013) by Newton Lee

Venkatraman Ramakrishnan interview: 'It takes courage to tackle very hard problems in science

Ben Stein interviewed by Paul Crouch Jr. on Trinity Broadcasting Network, First To Know with Paul Crouch Jr., April 21, 2008, 21 April 2008, 2011-12-19 http://www.tbn.org/video_portal/?which=bts,