The will to keep living... The resolve to change fate. Let's call this power... "Determination."
ENTRY NUMBER 5, True Lab
Determination is naturally possessed by humans. Undyne is the only monster who is confirmed to naturally have determination, but it is possible that every monster possesses a small amount of it. Should a monster acquire a high concentration of determination, their body melts, usually into an amalgamate. A being without a SOUL can gain the will to live if it acquires determination, a prime example being Flowey.
If the protagonist kills Undyne, then she melts in a similar fashion to the Amalgamates before turning to dust.
True Pacifist Route
In the True Lab, it is revealed that Alphys started experimenting with determination extracted from human SOULs in an attempt to unleash the power of the SOUL. She used blueprints to operate the DT Extraction Machine to obtain determination and began injecting it into the bodies of monsters who had fallen down and a Golden Flower that as a result became Flowey. However, since their bodies cannot handle determination, the monsters injected with determination soon melted together and became Amalgamates.
When using a SAVE Point after exhausting a region's kill count, it only says "Determination." with no context in particular.
When Undyne takes a blow from the protagonist, she holds on through her determination and challenges the protagonist as Undyne the Undying. Upon defeat, she melts in a similar fashion to how she melts on the Neutral Route before turning to dust.
- The SAVE Point description in Temmie Village refers to determination as "detemmienation."
- "Determination" might be a play on words, as the primary power it affords is the ability to reverse death, either the protagonist's or those they may have killed, hence de-termination.
- It is unknown if the Amalgamates mimicking game elements such as the protagonist's "!" dialogue box or SAVE Points are actually happening and related to the Determination they have been injected with, or are simply breaks of the fourth wall intended to catch players off-guard.
- The DT Extraction Machine resembles most parts of Photoshop Flowey (Most notably the inverted mouth and the eye sockets).
- Most of the text in Undertale is rendered in a font called Determination Mono.
- "ENTRY NUMBER 5 I've done it. Using the blueprints, I've extracted it from the human SOULs. I believe this is what gives their SOULs the strength to persist after death. The will to keep living. The resolve to change fate. Let's call this power... "Determination.""
- "And when I tried to load my save file... It didn't work. Your DETERMINATION! Somehow, it's even greater than mine!" - Flowey
- "You see, unlike humans', monsters' bodies don't have enough... Physical matter to take those concentrations of "determination." Their bodies started to melt, and lost what physicality they had. Pretty soon, all of the test subjects had melted together into... Those." - Alphys, True Lab
- "ENTRY NUMBER 8 I've chosen a candidate. I haven't told ASGORE yet, because I want to surprise him with it... In the center of his garden, there's something special. The first golden flower, that grew before all the others. The flower from the outside world. It appeared just before the queen left. I wonder... What happens when something without a SOUL gains the will to live?"
- "ENTRY NUMBER 1 This is it... Time to do what the King has asked me to do. I will create the power to free us all. I will unleash the power of the SOUL."