Prime Minister Lee Hsien Loong yesterday warned against Singapore's politics going down the path of polarisation, as this would divide the country and send it into a downward spiral.

Singapore will not just cease being an exceptional nation if politics permeates every issue and every subject becomes partisan, he added. "It will be the end of us."

In a speech setting out why it is crucial for the country to get its politics right, PM Lee said that having an adequate number of opposition MPs in Parliament is good for Singapore, as it keeps the Government on its toes and shows Singaporeans that it has nothing to hide.

But for politics here to work, the Government and the opposition must share the overriding objective of working for Singapore, and not just for their party or supporters.

Having more opposition MPs and fiercer debate in the House may not necessarily be better, the Prime Minister said, cautioning that the tone of Singapore's political debate could change for the worse.

"We all hope that diversity will make a hundred flowers bloom. But how do we prevent diversity from producing polarisation?" he said.

"How do we make sure that disagreement does not result in paralysis?" he asked, noting that this has happened in many other countries.

He expects the tone of debate in Parliament to shift with a stronger opposition presence, and said People's Action Party MPs will have to raise their game, be prepared for sharper questioning, and defend the Government's policies and decisions while speaking up for their constituents.

PM Lee called on opposition MPs to also step up and go beyond criticising government proposals to putting up their own proposals to be examined and debated.

The Government, on its part, will take an open and constructive approach, he said.

"On the specific details of policies... we will be open-minded, we will listen to the different voices. We can try different schemes, solutions. We will take in all constructive views and perspectives."

But on major issues concerning Singapore's fundamental interests, the Government cannot wait passively for consensus to form, he added. If there remain different views at the end of a full discussion, it will have to make the decision it judges best, and take responsibility.

"Having been elected to govern, we must govern," he said.

It is the Government's duty to make such decisions, and be accountable to Singaporeans for them, he added.

However, if the issue is not policies and priorities but a challenge to his team's fitness to govern, then the Government will have to stand up and defend itself vigorously, PM Lee said. "It must put down the challenge and prove that it deserves to be the government," he said. "Because otherwise, it must step aside and let another team take over."

Singapore's Westminster-style democracy, based on the British model, is inherently adversarial, PM Lee noted.

In Parliament, the leader of the opposition sits opposite the prime minister, challenging the incumbent and pointing out his faults, he said. The leader of the opposition highlights the Government's shortcomings and chips away at his opponents' credibility, with the ultimate goal of taking their place in government.

PM Lee noted that Mr Keir Starmer, the Labour Party's new leader, is doing his best to "show up current British PM Boris Johnson, and to make a name for himself". In Australia, question time for Prime Minister Scott Morrison happens every day when Parliament sits, often lasting more than an hour.

"Every encounter is a gladiatorial contest," he said.

In both these jurisdictions, the prime minister has to "stand his ground, defend his government's policies and maintain psychological dominance, to show that he deserves to be PM".

"If not, MPs on both sides will sense it, and so will the public, and this will influence election outcomes as well as leadership contests in their parties," PM Lee said.

In Singapore, the tone of parliamentary debate is less combative and its political traditions have developed differently, he noted.

PM Lee said he listened closely to Leader of the Opposition Pritam Singh's speech on Monday, on how he intends to perform his new role.

"I applaud his tone and approach," he said of the Workers' Party chief. "The government benches will do our part to work with him, to keep Parliament a constructive forum for debate."

Ultimately, the type of politics that Singapore has depends on Singaporeans, who have a responsibility to engage in public discourse, send the right signals at the ballot box and reward parties that do the right thing and deliver, he said.

"The standards they demand of political leaders, PAP and opposition, will influence the quality of the political leadership, the level of discussion and debate in Parliament; they will determine whether our politics enables us to thrive and prosper, or divides and destroys us," he added.

In Singapore, the PAP Government has been able to do the right things for Singaporeans and still get re-elected, PM Lee noted. "Sometimes we've paid the price in the vote, but overall, we've continued to win elections. And therefore the Government has been able to think long term, well beyond the next general election."

The country progresses, Singaporeans benefit and the PAP continues to win elections in a "virtuous, self-reinforcing circle".

"This model has worked well for Singapore," PM Lee said. "Once broken, it will be very difficult to put back together again."

He asked if Singapore could continue to work this way, and keep its focus on the long term with more diversity and contestation.

"At what point does a vote for a strong opposition become a vote for a different government?

"Is it really true that one day if there is a change of government, a new party can run Singapore equally well because we have such a good public service, as Mr Pritam Singh suggested on Monday?

"It's like saying you have the New York Philharmonic Orchestra, anybody can be the conductor."

These questions have no easy answers, and in the nature of politics and human societies, things can and do go wrong, PM Lee said.

"Each successive generation of Singaporeans has to keep on doing its best to keep the system working right," he said.

"The PAP feels acutely its special responsibility to keep on doing its best for Singapore, and keep Singapore working in this unique way. That is our sacred mission.

"We will do our utmost to persuade good men and women to enter politics, to take over the torch and lead the next generation forward. We will fight hard to win the hearts and minds of Singaporeans, and show Singaporeans that the PAP continues to deserve their support and trust."