Why does mocking not work when the mocked method is private and called from inside the class?

samplemock.php

<?php

class A
{
	public function doFirst()
	{
		return $this->doSecond();
	}
	
	private function doSecond()
	{
		return false;
	}
}

$a = new A();

$mock = Mockery::mock($a);
$mock->shouldReceive('doSecond')->andReturn(true);

var_dump($mock->doFirst()); //Expected: true, got false