Posted by Grant Richardson, 31st July, 2015
Coherence means how much logical sense your writing makes; how comfortably the ideas flow from one sentence or paragraph to the next. It pertains to the ‘structure’ and ‘content’ of a text or paragraph.
Cohesion means how smoothly those ideas are expressed and connected in words. It pertains to the use of language such as cohesive devices (linking words) and grammar.
For example, consider the following three sentences:
I have a headache. I didn’t sleep well last night. I have an important exam tonight.
It uses no cohesive devices but it is coherent. Adding cohesive devices may improve the readability, but it is not necessary.
Compare it with:
I have a headache. I normally fall asleep before midnight. My exam tonight starts at 7pm.
This has no coherence. Adding cohesive devices will not help. In fact, they will probably confuse the reader more (by forcing him/her to try to find a non-existent relationship between ideas).
Conclusion: cohesive devices will only improve writing if it already has coherence. Adding cohesive devices without coherence will not improve the coherence, but may worsen it.