Perhaps the key experiment in the history of modern science is Galileo's demonstration that falling objects must fall at the same rate regardless of their masses. This is widely thought to have been a straightforward physical demonstration, involving climbing up the Leaning Tower of Pisa and dropping two heavy weights off it, whereas in fact, it was a logical demonstration, using the 'thought experiment' technique. The 'experiment' is described by Galileo in "Discorsi e dimostrazioni matematiche" (1638) (literally, 'Discourses and Mathematical Demonstrations') thus: