Object permanence

Object permanence, a period of rapid cognitive development is often linked to separation anxiety. Starting at 6-8 months it becomes permanent at 16-18 months.

In the first year, babies are discovering everything has a life of its own even if it is out of sight. Objects puzzlingly seem to appear, disappear and reappear.

https://www.simplypsychology.org/sensorimotor.html

https://youtu.be/TRF27F2bn-A

Egocentricism

Babies are extremely egocentric with no understanding of the world other than their own current point of view. During this stage the baby's understanding is, objects exist and events occur in the world independently of their own actions ('the object concept', or 'object permanence').

Mental Representation

Object permanence means knowing an object still exists, even if it is hidden out of sight. It is requiring the ability (on the baby's part) to form a mental representation of the object.
For example, placing a toy under a blanket, the baby who has achieved object permanence knows it is still there and can actively look for it. At the beginning of this stage, the baby is behaving as if the toy has simply disappeared. The attainment of object permanence is signalling the transition to the next stage of development (preoperational).

The 6 Stages of Object Permanence & Separation Anxiety

https://wehavekids.com/parenting/The-Six-Stages-of-Object-Permanence

Babies at 6-8 months can often cry for no particular reason, and it can be troubling for a parent. It's important to be aware, as a parent, you don't need to fix all their emotional crying. It's a stress release for the baby and is cathartic. This can be quite a challenging concept for a parent to wrap their head around.

http://www.awareparenting.com/answer27.htm

Aletha Solther, an eminent Psychologist, advises parents to (once all needs are met) accept babies may cry for a time while being held, rocked and/or verbally reassured. The crying can then become productive by building trust. While allowing babies and little ones to process big emotions in the only way they are capable.

Suppressing emotions

Suppressing emotions is counterproductive and can actually be harmful, research has found. It's refusing to allow the baby to have a voice and be heard. Once this happens, it risks becoming a control pattern. It is actually quite harmful, and the baby absorbs their stress. For example, a crying baby is put on the breast or a dummy placed in their mouth; it is saying to the baby, your emotions are not valid.

Never Allow Babies To Cry Alone

As Aletha Solter says, "One of the roles of parents is to listen and empathize with babies and children. You cannot always "fix" everything for your baby, but you can help by being there, holding, listening, and letting her know you love her". The key thing to remember is we NEVER ALLOW BABIES TO CRY ALONE. Allow your baby to cry in your arms as long as the baby needs to (after all immediate needs have been met). After the baby is allowed to "catch up" on the crying they need to do, they will probably become much less demanding and fussy.