So there are spells like fire bolt that specifically say you can target a creature or object. Then there's spells like sacred flame that just say creature.
RAW, can I use a damaging spell on an object, even if it only says creature?
RAI I'm pretty sure that guiding bolt (a blast of holy energy) could target anything, whether or not it is a creature or object.
A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below). (PHB)
the spell will tell you what you can target with it. The choices are intentional
As stated above, what you can target is specified by the spell. So to answer your question, no, RAW - you cannot cast Guiding Bolt on an object.
And I'd also say RAI, you can't either. This is a spell designed specifically to smite creatures, and then (if they don't die), make them more easily smiteable next time. The reasoning your deity grants this power that works in this way, from a narrative standpoint, is up to you.
As stated the spell description will indicate whether you target a space or a creature. Call Lightning specifically states a point: "When you cast the spell, choose a point you can see under the cloud. A bolt of lightning flashes down from the cloud to that point."
At that point the DM decides what damage is done to anything or creature within 5 feet of the targeted point. If the bridge is stone, I would expect the DM to decide no damage, if it is a major wood bridge, still nothing. A minor foot bridge, still would be debatable. picture a character at this scene casting call lightning: https://youtu.be/Y-LPERlRHYA
The lightning strikes the tree, but the tree is not suddenly destroyed and or aflame. It depends on how dense the wood is. I would say that a rope bridge would ignite because it is more dry, but all up to the DM discretion on the amount of damage. There is always the possibility of the DM using the hardness rules of the material of the bridge and then asking the player to roll damage and any damage after hardness might ignite. Makes sense to me. All of the DMS that I have played with in the past would make the call on the fly as to not interrupt play.
But to your original point, the description says creature or object or a point in space. Since call lightning says point in space, yes it could possible damage a bridge.
Almost everything that says "attack" can be used against objects.
Almost all Fire spells are generally considered to light flamable things on fire, as long as they 'activate'. Fire Shield only activates when you are hit, but Flame Blade is a melee attack.
In the strictest sense iconarising is correct and a spell will tell you if it can target an object or not.
However, there are also general rules for destroying objects; basically it boils down to the DM deciding if you can, and what it might take, with sample values for AC, hit-points etc. A thin rope might just require a hit with enough damage to cut through it in one go, but a thick rope might require multiple turns of sawing away at it to lower a drawbridge or whatever.
So most accurate answer is "you can target an object if the spell says so, but your DM might rule it is unaffected, otherwise your DM can decide what you can hit and damage with your spell". This is because objects encompass such a wide variety of potential targets with so many different material compositions etc. that you just can't have a rule that says yes or no. For example, you could light bedsheets on fire with a torch, but they're going to laugh off your attempts to punch them apart, but you can absolutely slice them, cold probably won't do much etc. etc.
Almost everything that says "attack" can be used against objects.
Almost all Fire spells are generally considered to light flamable things on fire, as long as they 'activate'. Fire Shield only activates when you are hit, but Flame Blade is a melee attack.
This is not generally true - the vast majority of spells with a to hit roll require them to target creatures.
Eldritch Blast, Ray of Frost, Chill Touch, Produce Flame, Shocking Grasp, Thorn Whip, Vicious Mockery, Chaos Bolt - all these cantrips target creatures only.
The only cantrip that explicitly allows targeting of objects is Fire Bolt. The ammunition produced by Magic Stone could also be used to target an object.
Out of all the remaining spells that require an attack roll in the game (about 20) - only the following do not explicitly target creatures:
Melf's Acid Arrow (2): "a target within range"
Flame Blade (2): " melee spell attack with the fiery blade."
Scorching Ray(2): " at targets within range"
Crown of Stars (7): "one creature or object "
Mordenkainen's Sword (7): " against a target of your choice"
Blade of Disaster(9): "each one against a creature, loose object, or structure "
P.S. DMs can run things differently for their games of course since it is easy to ask WHY specific spells can only target creatures.
One answer could be that the magic of the spell draws part of its power from the life force of the target and so are completely ineffective against non-living targets. Alternatively, the magic isn't strong enough to affect objects but is more effective against creatures. There are many possible answers. Rules wise the spells indicate what they can damage/affect/be targeted at - but it is up to the DM to decide how things work in their world and whether those constraints are appropriate for how they consider magic to work in their world.
Another example, are lightning bolt and fireball - these spells do a lot of damage to creatures and may light flammable objects on fire but otherwise they do not damage objects. You might be able to set a wooden bridge on fire but you wouldn't be able to blown one up with either spell - RAW, they don't do that sort of explosive damage even though many people imagine a fireball as an explosion.
So there are spells like fire bolt that specifically say you can target a creature or object. Then there's spells like sacred flame that just say creature.
RAW, can I use a damaging spell on an object, even if it only says creature?
RAI I'm pretty sure that guiding bolt (a blast of holy energy) could target anything, whether or not it is a creature or object.
Targets
A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below). (PHB)
the spell will tell you what you can target with it. The choices are intentional
As stated above, what you can target is specified by the spell. So to answer your question, no, RAW - you cannot cast Guiding Bolt on an object.
And I'd also say RAI, you can't either. This is a spell designed specifically to smite creatures, and then (if they don't die), make them more easily smiteable next time. The reasoning your deity grants this power that works in this way, from a narrative standpoint, is up to you.
so I can't call lightning on a bridge to weaken/destroy it?
As stated the spell description will indicate whether you target a space or a creature. Call Lightning specifically states a point: "When you cast the spell, choose a point you can see under the cloud. A bolt of lightning flashes down from the cloud to that point."
At that point the DM decides what damage is done to anything or creature within 5 feet of the targeted point. If the bridge is stone, I would expect the DM to decide no damage, if it is a major wood bridge, still nothing. A minor foot bridge, still would be debatable. picture a character at this scene casting call lightning: https://youtu.be/Y-LPERlRHYA
The lightning strikes the tree, but the tree is not suddenly destroyed and or aflame. It depends on how dense the wood is. I would say that a rope bridge would ignite because it is more dry, but all up to the DM discretion on the amount of damage. There is always the possibility of the DM using the hardness rules of the material of the bridge and then asking the player to roll damage and any damage after hardness might ignite. Makes sense to me. All of the DMS that I have played with in the past would make the call on the fly as to not interrupt play.
But to your original point, the description says creature or object or a point in space. Since call lightning says point in space, yes it could possible damage a bridge.
Almost everything that says "attack" can be used against objects.
Almost all Fire spells are generally considered to light flamable things on fire, as long as they 'activate'. Fire Shield only activates when you are hit, but Flame Blade is a melee attack.
In the strictest sense iconarising is correct and a spell will tell you if it can target an object or not.
However, there are also general rules for destroying objects; basically it boils down to the DM deciding if you can, and what it might take, with sample values for AC, hit-points etc. A thin rope might just require a hit with enough damage to cut through it in one go, but a thick rope might require multiple turns of sawing away at it to lower a drawbridge or whatever.
So most accurate answer is "you can target an object if the spell says so, but your DM might rule it is unaffected, otherwise your DM can decide what you can hit and damage with your spell". This is because objects encompass such a wide variety of potential targets with so many different material compositions etc. that you just can't have a rule that says yes or no. For example, you could light bedsheets on fire with a torch, but they're going to laugh off your attempts to punch them apart, but you can absolutely slice them, cold probably won't do much etc. etc.
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This is not generally true - the vast majority of spells with a to hit roll require them to target creatures.
Eldritch Blast, Ray of Frost, Chill Touch, Produce Flame, Shocking Grasp, Thorn Whip, Vicious Mockery, Chaos Bolt - all these cantrips target creatures only.
The only cantrip that explicitly allows targeting of objects is Fire Bolt. The ammunition produced by Magic Stone could also be used to target an object.
Out of all the remaining spells that require an attack roll in the game (about 20) - only the following do not explicitly target creatures:
Melf's Acid Arrow (2): "a target within range"
Flame Blade (2): " melee spell attack with the fiery blade."
Scorching Ray(2): " at targets within range"
Crown of Stars (7): "one creature or object "
Mordenkainen's Sword (7): " against a target of your choice"
Blade of Disaster(9): "each one against a creature, loose object, or structure "
P.S. DMs can run things differently for their games of course since it is easy to ask WHY specific spells can only target creatures.
One answer could be that the magic of the spell draws part of its power from the life force of the target and so are completely ineffective against non-living targets. Alternatively, the magic isn't strong enough to affect objects but is more effective against creatures. There are many possible answers. Rules wise the spells indicate what they can damage/affect/be targeted at - but it is up to the DM to decide how things work in their world and whether those constraints are appropriate for how they consider magic to work in their world.
Another example, are lightning bolt and fireball - these spells do a lot of damage to creatures and may light flammable objects on fire but otherwise they do not damage objects. You might be able to set a wooden bridge on fire but you wouldn't be able to blown one up with either spell - RAW, they don't do that sort of explosive damage even though many people imagine a fireball as an explosion.