• not <expression> returns the opposite of the expression’s value: it turns true into false and false into true.
• <expression> and <expression> returns true if both expressions’ values are true, and false if either is false.
• <expression> or <expression> returns true if either expression’s value is true, and false if both are false.

@debug not true; // false
@debug not false; // true

@debug true and true; // true
@debug true and false; // false

@debug true or false; // true
@debug false or false; // false

@debug not true  // false
@debug not false  // true

@debug true and true  // true
@debug true and false  // false

@debug true or false  // true
@debug false or false  // false

Truthiness and Falsiness

Anywhere true or false are allowed, you can use other values as well. The values false and null are falsey, which means Sass considers them to indicate falsehood and cause conditions to fail. Every other value is considered truthy, so Sass considers them to work like true and cause conditions to succeed.

For example, if you want to check if a string contains a space, you can just write string.index($string, " "). The string.index() function returns null if the string isn’t found and a number otherwise.