utilisation sans main

A.2 - Activated Abilities
A.2.1 - Activated abilities are written as ": " when the ability is on a permanent in play, or on a card which is in a zone other than "in play" that also has text such as "Play this ability only if is in ." [CompRules 1999/04/23]

G.6 - Costs
G.6.1 - Spells and activated abilities have a cost you must pay when playing that spell or ability. This cost is paid during announcement. In addition, you may be instructed to pay costs during the resolution of a spell or ability. [CompRules 1999/04/23]
G.6.2 - The most common costs are mana payments, tapping a permanent (see Rule G.38), payment of life (see Rule G.19), and sacrifices (see Rule G.33), but a cost can be anything. [CompRules 1999/04/23]
G.6.3 - The payment of a cost is unpreventable if you have the required resources (most often this is mana). Effects may prevent you from having the resource available. For example, the cost may be raised by Gloom or a card may not be tappable due to Volrath's Curse. [D'Angelo 1996/11/07]
G.6.4 - You cannot pay a cost if you do not have the resource available. [CompRules 1999/04/23]

Null Brooch

Color= Artifact Type= Artifact Cost= 4 EX(R)
Text (EX+errata): {2},{Tap},Discard your hand: Counter target noncreature spell. [Oracle 1999/05/01]

You can play the ability if you have no cards in hand. [Barclay 1998/06/10]
A "creature spell" is any "Creature" or "Artifact Creature" spell. [D'Angelo 1999/06/01] Older cards of type Summon are also Creature cards.