Mencius - Book 4, Part 1, Chapter 1A

Mencius said, 'The power of vision of Lî Lâu, and skill of hand of Kung-shû, without the compass and square, could not form squares and circles. The acute ear of the music-master K'wang, without the pitch-tubes, could not determine correctly the five notes. The principles of Yâo and Shun, without a benevolent government, could not secure the tranquil order of the kingdom.

'There are now princes who have benevolent hearts and a reputation for benevolence, while yet the people do not receive any benefits from them, nor will they leave any example to future ages; all because they do not put into practice the ways of the ancient kings.

'Hence we have the saying: "Virtue alone is not sufficient for the exercise of government; laws alone cannot carry themselves into practice."

It is said in the Book of Poetry,

"Without transgression, without forgetfulness,
Following the ancient statutes."

Never has any one fallen into error, who followed the laws of the ancient kings.

'When the sages had used the vigor of their eyes, they called in to their aid the compass, the square, the level, and the line, to make things square, round, level, and straight: the use of the instruments is inexhaustible. When they had used their power of hearing to the utmost, they called in the pitch-tubes to their aid to determine the five notes: the use of those tubes is inexhaustible. When they had exerted to the utmost the thoughts of their hearts, they called in to their aid a government that could not endure to witness the sufferings of men: and their benevolence overspread the kingdom.
~ James Legge translation via nothingistic.org ~

Go here to read the introductory post to this serialized version of the Works of Mencius.