Show me somewhere in the PHB or DM's Guide about being able to target objects with spells such as Fireball.
I (and others) quoted it upthread:
PHB p 55
Fire: Explosive bursts, fiery rays, or simple ignition.
DMG pp 65-66
Damaging Objects
Like characters, objects have hit points and defense scores (except for Will defense; see Object Immunities and Vulnerabilities, below). . .
Object Immunities and Vulnerabilities
Usually, it doesn’t matter what kind of attack you make against an object: Damage is damage. However, there are a few exceptions.
All objects are immune to poison damage, psychic damage, and necrotic damage.
Objects don’t have a Will defense and are immune to attacks that target Will defense.
Some unusual materials might be particularly resistant to some or all kinds of damage. In addition, you might rule that some kinds of damage are particularly effective against certain objects and grant the object vulnerability to that damage type. For example, a gauzy curtain or a pile of dry papers might have vulnerability 5 to fire because any spark is likely to destroy it.
the argument was that as originally written a player could not choose to target objects in D&D 4e unless object was listed as a valid target.
<snip>
Some posters <snip> appear to be arguing that one has always been able to attack and damage objects in the original rules as published (without errata) because of various reasosn including... the fire keyword or the damaging objects rules in the DMG.
Yes. I am arguing that rules for the effect of attacks upon objects count as rules for attacking objects. I don't know what you thought those rules were for - maybe they just had a few blank pages they needed to fill?
There was also p 42, just in case anyone was in doubt as to whether the GM has authority to adjudicate fictional positioning:
DMG p 42
Your presence as the Dungeon Master is what makes D&D such a great game. You make it possible for the players to try anything they can imagine. That means it’s your job to resolve unusual actions when the players try them.
I can pretty easily imagine setting things alight with a spell that causes explostive bursts, fiery rays and/or simple ignition! And the GM even has some additional guidelines besides p 42, namely those I quoted from pp 65-66.
How much discussion is there of damaging objects in Moldvay Basic? That's right,
none. People still managed to adjudicate it and I have
never heard it argued that in Basic you cannot damage objects or set them on fire. The difference with 4e - 4e actually has a page or more devoted to the topic! That's a strange form of "no rules".