A man constrained by conscience...

Who in the world
is a man constrained by conscience,
who awakens to censure
like a fine stallion to the whip?
Those restrained by conscience
are rare — 
those who go through life
always mindful.
Having reached the end
of suffering & stress,
they go through what is uneven
evenly;
go through what is out-of-tune
in tune.

Hiri Sutta-Conscience
http://www.accesstoinsight.org/tipitaka/sn/sn01/sn01.018.than.html

Note:

The Pali word "Hiri" is translated as conscience here.  This is also translated
 as shame and is compared to a moat in Nagara sutta (the fortress)... see below:

"Just as the royal frontier fortress has
a moat, both deep & wide, for the protection of those within and to
ward off those without; in the same way, the disciple of the noble ones
has a sense of shame. He feels shame at [the thought of engaging in]
bodily misconduct, verbal misconduct, mental misconduct. He feels shame
at falling into evil, unskillful actions. With shame as his moat, the
disciple of the noble ones abandons what is unskillful, develops what
is skillful, abandons what is blameworthy, develops what is blameless,
and looks after himself with purity. With this second true quality is
he endowed.
Nagara Sutta

Abandoning shamelessness is also a initial step in the path that leads to abandon birth, old age, death (enlightenment) according to Thyodhamma sutta: please see the post below for more details:

"One bad Brick" - The fault finding mind

To listen to MP3 of Thyodhamma Suttahttp://www.gautamabuddha.ca/bana/english/14_Kitariri_Sutta.MP3(In the link it should read Thayodhamma sutta, not Kitagiri sutta)

Definition From "A Glossary of Pali and Buddhist Terms"
hiri-ottappa [hiri-ottappa]: "Conscience and concern"; "moral shame and moral dread." These twin emotions — the "guardians of the world" — are associated with all skillful actions. Hiri is an inner conscience that restrains us from doing deeds that would jeopardize our own self-respect; ottappa is a healthy fear of committing unskillful deeds that might bring about harm to ourselves or others.

http://www.accesstoinsight.org/glossary.html