How to specify the beginning of the month in Postgres SQL syntax?

Hello,

I need to create a view in Postgres that has a where clause of the
date < beginning of month.

i.e.:
SELECT supplier_number, Sum(amount) AS due
FROM purchase_orders
WHERE date < '2003-12-1' AND paid = 0
GROUP BY supplier_number
ORDER BY supplier_number ASC

As you can see, I've specified the 1st of December this year as the
where clause. What I want is an SQL statement that automatically
generates the first of the month. How do I do this?

Thanks.