When thinking about the pursuit of knowledge, it might be easy to assume that there are no limitations or constraints to this pursuit at all. However, when looking at this question more closely, you will find that in fact there are some constraints that impact on the successful pursuit of knowledge. Let me highlight some of these constraints for you.
Firstly, you might want to look at practical constraints. Someone might want to increase their knowledge about a certain topic, but they might not have the actual physical means to do so. For example, they might not have the equipment needed to do the research they would like to do. Perhaps the necessary equipment hasn't even been invented yet, therefore limiting the success of this research severely.
You could also mention financial restraints. Whilst it might be very interesting to pursue the answer to a certain question in order to increase one's knowledge, one might not have the financial means to finance the actions needed in order to undertake the required research. In this case, it is an option to try and find a sponsor to help with the finances, but if this search for a sponsor is unsuccessful, the pursuit of knowledge will not be able to go ahead in this instance.
Lastly, you might want to mention ethical and moral limitations. For example, whilst it might be tempting to experiment on humans to find answers to some pressing medical questions, there is a limitation to the extent to which this kind of research is ethically and morally justifiable. Anything that puts a person's health or life at risk is clearly not acceptable practice for ethical and moral reasons, which could therefore also pose a limitation to the pursuit of knowledge.