It's equally hard to know what "laissez faire" means in this context. 
For example, by 1922, his second year as Secretary of Commerce, 
Herbert Hoover convened a conference on unemployment and the business 
cycle that drew heavily on the work of Otto Mallery, a 
proto-Keynesian who prescribed countercyclical spending to smooth out 
the business cycle.  Hoover clearly understood the relation between 
aggregate demand and national income much as Keynes came to 
understand it and, after this conference, was a strong advocate 
through the 20s of voluntary countercyclical investment by firms.  As 
Secretary of Commerce and then as President after 1929, he 
consistently urged firms to put these and other macroeconomic 
stabilizers (like unemployment insurance) in place, but always 
resisted calls to enforce this by law or, as Keynes would propose in 
1933, have the government do the countercyclical borrowing and 
spending itself.  He was, that is, a convinced "Keynesian," or at 
least a fellow traveler, in everything but Keynes's crucial 
conclusion, that government must be the borrower and spender of last 
resort.  Perhaps like Krugman, I was taught in high school American 
history (circa 1963) that Hoover was the discredited avatar of 
laissez faire and that the New Deal brought Keynes to America in the 
30s.  The second proposition, to the continuing surprise of my 
students, is clearly false; the first depends on what exactly one 
means by "laissez faire;" does Hoover's faith in the persuasive power 
of the jawbone wielded from the bully pulpit qualify as 
"intervention" into the economy?

Rich Adelstein