Flight under Visual Flight Rules (VFR) is very restricted. For this reason Instrument Flight Rules (IFR) have been established, to allow a properly trained pilot to safely conduct a flight through restricted visibility and cloud.
Some of the rules
Minimum Height Rule. Inside and outside controlled airspace and aircraft flying IFR must not fly below altitude 1,000ft above the nearest obstacle within 5nm, unless stated by ATC. When flying above 3,000ft or above the appropriate transition altitude, the Quadrantal Rule should be adopted.
Flight Plan And ATC Clearance
Rule 35 applies to IFR flights within controlled airspace. For a flight to comply with IFR, before any flight in this condition, a flight plan must be filed either in the air or on the ground. An IFR clearance must be obtained and flight to be carried out specifically to that clearance. A pilot flying inside controlled airspace must fly the instructions given by the controller. IFR can be cancelled at any time providing the flight can be conducted under VFR conditions.