सज्जन refers to one who displays truthful conduct while दुर्जन is one who is dishonest. In the Adiparva of the Mahabharata, there is an episode where Shankuntala comes to Dushyanta with their child Bharata but Dushyanta refuses to recognise her. Shakuntala is rightfully upset with this behaviour and castigates him severely for his bad conduct. One of the points she brings up during her engagement with Dushyanta is what she believes must be the conduct of a सज्जन as against that of a दुर्जन and thereby implying that the behaviour of Dushyanta is akin to that of a दुर्जन. It becomes clear to anyone reading the below that human psychology today works in the same manner as it had worked thousands of years back when this event may have taken place.

विरूवो यावदादर्शे नात्मान: पश्यते‌ मुखम्। मन्यते तावद् आत्मानं अन्येभ्यो रूपवत्तरम्।।

An ugly person, until he sees his face in the mirror, considers himself more handsome than others.

यदा स्वमुखमादर्शे विकृतं सोSभिवीक्षते। तदान्तरं विजानीते आत्मानं चेतरं जनम्।।

But when he sees his own face in the mirror, it is then that he perceives the difference between himself and others.

अतीवरूप संपन्नो न कंचिदवमव्यते। अतीव जल्पन् दुर्वाचो भवतीह‌ विहेठक:।।

He who is.really handsome never taunts others. He, who taunts others too much, is only considered to be a reviler (or one who injures another).

मूर्खो हि जल्पतां पुंसा श्रुत्वा वाच: शुभाशुभा:। अशुभं वाक्यमादत्ते पुरीषमिव सूकर:।।

Just as a pig seeks for dirt and filth even in a flower garden, so does a मूर्ख see or imbibe impure out of the good and bad spoken by others.

प्राज्ञस्तु जल्पतां पुंसा श्रुत्वा वाच: शुभाशुभा:। गु़णवद् वाक्यमादत्ते‌ हंस: क्षीरामिवाम्भस:।।

Just as a हंस always is able to extract milk even though the milk is mixed with water, so does a wise man accept only what is good from a speech that is mixed with pure and impure words.

अन्यान् परिवदन्  साधुर्यथा हि परितप्यते। तथा परिवदन्नन्यांस्तुष्टो भवति‌ दुर्जनानां:।।

सज्जन always feel the pain to speak ill of others but दुर्जन always derive pleasure in doing the same thing.

अभिवाद्य यथा वृद्धान् सन्तो गच्छन्ति निर्वृतिम्। एवं सज्जनमाक्त्रुष्य मूर्खो भवति‌ निर्वृत:।।

सज्जन always feels pleasure in showing respect towards the old but a मूर्ख will always feel pleasure in abusing them. 

सुखं जीवन्त्यदोषज्ञा मूर्खा दोषानुदर्शिन:। यत्र वाच्या: परै: सन्त: पुरानाहुस्तथाविद्यान्।। 

सज्जन always feels pleasure in not seeking faults of others but a मूर्ख will feel happy doing this. A मूर्ख will always speak ill of honest men but an honest man will never injure a मूर्ख even if injured by them.

अतो हास्यतरं लोके किंचिदन्यन्न विद्यते। यत्र‌ दुर्जनमित्याह‌ दुर्जन: सज्जनं स्वयं।। 

What could be more ridiculous in the world than this – that those who are दुर्जन should address persons who are सज्जन as दुर्जन. 

सत्यधर्माच्युतात् पुंस: ऋुद्धादाशीविषादिव। अनास्तिकोSप्युद्विजते जन: किं पुनरास्तिक:।।

Even a नास्तिक‌ is afraid of those who have fallen from pursuit of सत्य & धर्म as all men are afraid of snakes of virulent poison. (When such is the case with a नास्तिक‌), what shall I speak of me who is an आस्तिक‌?

Source:

Translation of Mahabharata by MN Dutt