There are few decisions Holyrood can take that are bigger than a vote on assisted dying.
Whether or not to allow terminally ill adults to seek medical help to end their lives is of fundamental consequence.
The vote on Tuesday will also be of note to decision makers at Westminster, where similar proposals for England and Wales are working their way through the UK parliament.
MSPs have twice before rejected attempts to change the law, most recently in 2015.
The Liberal Democrat MSP Liam McArthur, who has developed the current proposal, believes the mood in parliament is now "very, very different".
He clearly expects his bill to have more support than previous attempts – but that is not the same as saying that he is sure it has sufficient support to proceed.
Speaking on BBC Scotland's The Sunday Show, he said he expected the first vote to be "close".
He could be concerned in part about the potential impact of recent interventions.
First Minister John Swinney has not changed his position over the years.
He has consistently voted against assisted dying and it was no surprise when he revealed that he intends to do so again.
He said that representations in favour of assisted dying from constituents who have witnessed loved ones suffer terribly at the end of their lives caused him to agonise over his decision.
However, he believes that changing the law would unacceptably alter the relationship between doctors and their patients and that it could put pressure on sick and disabled people to end their lives prematurely.
Swinney has also been careful to stress that he has reached a personal view, that he is not seeking to influence others, and the Scottish government will remain "neutral".
