The statement that political parties and pressure groups are 'sine qua non' (essential conditions) of democracy holds significant truth, as these entities play indispensable roles in the functioning and vitality of democratic systems. While often criticized, their absence would fundamentally alter, and likely undermine, the very nature of representative governance.

Political Parties: Political parties are organized groups that share common ideological perspectives and seek to gain and maintain political power through elections. Their essential functions in a democracy include:

1. Representation and Aggregation of Interests: Parties articulate and aggregate diverse societal interests into coherent policy platforms, providing citizens with a choice of visions for governance. They act as a crucial link between the populace and the government.
2. Mobilization and Participation: They mobilize voters, encourage political participation, and educate the public on issues, thereby enhancing democratic engagement.
3. Government Formation and Accountability: Parties form governments, provide leadership, and offer a mechanism for holding elected officials accountable. The prospect of losing power incentivizes parties to perform well.
4. Policy Formulation: They develop and implement public policies, translating broad ideological goals into concrete actions.
5. Conflict Management: By channeling diverse demands into the political process, parties can help manage societal conflicts and build consensus.

Without political parties, elections would be a chaotic contest of individuals, lacking coherent platforms or mechanisms for collective action and accountability. The ability to form stable governments and provide a clear choice to the electorate would be severely hampered.

Pressure Groups (Interest Groups): Pressure groups are organizations that seek to influence public policy without directly seeking to govern. They represent specific interests (e.g., business, labor, environmental, human rights) and exert influence through lobbying, advocacy, and public campaigns. Their democratic contributions include:

1. Articulation of Specific Interests: They provide a voice for particular segments of society whose concerns might not be fully captured by broad party platforms.
2. Policy Input and Expertise: Pressure groups often possess specialized knowledge and can provide valuable information and technical expertise to policymakers, enriching the policy-making process.
3. Checks and Balances: They act as watchdogs, scrutinizing government actions and holding power accountable, often highlighting issues that might otherwise be overlooked.
4. Enhancing Participation: They offer alternative avenues for citizens to participate in politics beyond voting, allowing for more focused advocacy on issues they care deeply about.
5. Pluralism: The existence of multiple, competing pressure groups ensures a pluralistic political landscape where no single interest dominates entirely.

While pressure groups can sometimes be criticized for promoting narrow self-interests or for their unequal access to power, their role in ensuring that a wide array of voices is heard in the policy-making process is vital. They complement the representative function of political parties by providing more granular input.

In conclusion, both political parties and pressure groups are fundamental to the health and functioning of modern democracies. Parties provide the structure for governance and broad representation, while pressure groups ensure that specific interests and concerns are articulated and considered. Together, they facilitate participation, accountability, and the dynamic interplay of interests that characterize a vibrant democratic system.